What is SAP and its basic purpose? How important SAP experience is to gain senior roles in corporate sector?

SAP is an Enterprise Resource Planning (ERP) software.
ERPs provide solutions to business and more comprehensive (detailed) report for analysis and decision making.
But it all depends on the data we entered into ERPs.
Effective decision making is based on the quality of data entry.
SAP experience is important to gain senior roles in corporate sector due to following aspects: It provides executive summary (fast and reliable information) on single click.
It provides comprehensive downloads in other user friendly applications such as MS Excel.
It provides central point for management of huge data.
It eliminates duplication of work.
(If data is enter at one point then it can be viewed at many other places such as employees information entered by HR can be viewed and used by Payroll department for processing of salaries and also by higher ups for analysis).
In short if you have the basic knowledge of SAP or any other ERP then at senior level you just need to focus on the source of data which is being provided for analysis and reports which are being generated....

As has been pointed out SAP is an ERP software by SAP AG, Germany.
It is one product which when implemented properly can handle a organisation's HR functions, Sales, Marketing, Product Management, Plant maintenance and much more.
Since all these are done via a single solution, the organisation gains insight from across departments and collaboration & control becomes easy.
However SAP experience is NOT a criteria to gain 'Senior' roles.
However what you mean by 'Senior' is not clear.
And SAP knowledge by itself is not going to get anyone a senior position either.
Do understand that SAP is one of the ERP products and a leader too.
However there are other ERP solutions in the market from Oracle, Microsoft and others.

If you are capable to understand how to plan with the available resources in your organisation, then it is what all the ERP software does. SAP is the market leader of the many  ERP software packages available in the market. This has got nothing to do with a senior role in a corporate sector. There are so many other criteria that will lead to a senior roles.