How do you do estimation in Agile? how to estimate using planning poker?

Estimation is usually based on story points for user stories and hours/effort for tasks derived from those story points.
In poker planning, team members ask product owners questions about the story and what's needed to be done to help them determine how complex the story is compared to a reference story.
The planning can use poker chips where each team member throw their chips based on how they think the story is complicated compared to others or they can use fibonnaci series to vote to determine the story's points worth based on a reference story with a predefined story weight.

Planning Poker: A consensus-based estimation technique, mostly used to estimate effort orrelative size of user stories in agile software development. It is a variation of the WideBand Delphi method using a deck of cards with values representing the units in which theteam estimates.