In PM world, their was a concept called (Uncertainty). Can you explain the meaning of this concept and it's influences?

As it was said, uncertainty is always there in projects execution. It can be reduced by various ways but risk mitigation plan is the most important factor in such issues.

Uncertainty is always exist although you deployed a lot of risk mitigation tools due to the unknown-unknowns risks.

The Means of  uncertainty refers to uncertainty around how the product will be built or lack of clarity about the project execution. Means uncertainty can be reduced through acquiring more knowledge about the project

Uncertainty is always there in projects to a certain level . there are various ways how this can be reduced. some of them are controllable and some of them are not. 

Those risk factors that may occur due to controllable factors can be mitigated while the others can not. Risk can be avoided by putting in place a risk mitigation policy as well as effective forecasting techniques can be introduced

Once we have reduced uncertainty to a practical minimum, the only other thing we can do is to take the remaining uncertainty into account in our process. We’ll look at strategies suited for projects that are subject to different kinds of constraints below. (For simplicity, we’ll assume that resources are fixed, since the ability to add resources does not vary in a meaningful way between the different scenarios. Similarly, we also assume that scope is well-enough controlled to prevent scope creep.)

Fixed Schedule, Fixed Scope

The first thing to understand about this set of constraints is that success is not always possible. If scope is truly fixed, and schedule is subject to uncertainty, then we have already seen that extreme cases will break any schedule.

These projects are planned with uncertainty in mind, by adding enough buffer time into the schedule to handle a reasonable level of uncertainty (for example, allocate30% of the schedule for this purpose). This approach works well in situations where uncertainty is small, such as for repeating processes (e.g., laying carpet, or painting houses).

Fixed Schedule, Adjustable Scope

Many agile project-management frameworks handle uncertainty by committing to the only thing that can truly be controlled, which is the schedule, and adjusting the scope as required to meet the schedule. This strategy is particularly useful in high-uncertainty environments, where estimation is known to be inaccurate, and where scope is not well-understood and may change frequently.

The Scrum framework handles this situation effectively. It requires careful planning, but in a way that handles high uncertainty gracefully. Scrum projects work in short cycles to deliver modest increments of scope quickly, and to allow for frequent changes in scope and priority. Within each cycle, scope is adjusted as necessary in order to guarantee that the schedule is met.

No Schedule, Unknown Scope

Uncertainty is greatest when the scope is not known prior to the start of execution. This is the case for reactive organizations, such as Customer Support groups, which receive urgent requests that must be handled quickly, but which cannot be scheduled or planned in any meaningful way.

Another agile process, Kanban, is useful for this scenario. Kanban processes re-prioritize requests daily, and throttle (control) the flow of work by limiting simultaneous work-in-progress to a specified number (e.g., up to three concurrent requests can be handled by the staff).

Uncertainties means the unknown risks that may be required to be included in the Risk Management Plan and a certain amount of costs by way of "Contingencies" is added to the Project Cost when arriving at the overall Budgeted Cost of the project.  

Uncertainty is a term attached to everything by default. There are internal factors as well as external factors and one should learn to minimise it by focusing on the positive side . Factors within control should be dealt amicably and factors beyond control should be dealt with calculated risk method.

Thanks for the invitation

SO in my point of view ....uncertainty is always there

in projects execution, performance,carrying out  & in the

same time it,s the most big challenges of PM too.

Uncertainty is the big challenge of PM, it is the shortage of information.

All of us can manage when the information is avaliable, but who can manage when it is not?

it is the case of forecasting and how to make the required analysis to suppose the amount of contingency

Uncertainty can occur any time in projects specially in large scale. I was associated in project management of one cement plant. At the time of start of project, the management approved the cost for imported plant & machinery, which overrun due to depreciation of dollar. As a result, management arranged extra equity and obtained extra loan from lenders which affect on project cost as well as projections of cash flow. 

Uncertainty is a lack of knowledge about an event related to cost, time, the quality needs, communications needs, etc., can be uncertain. The investigation of uncertainties may help identify risks,although initiation process has the greatest degree of uncertainty(higher risk)