Drill in WEBI Reports....?

Hi everyone,

     I'm santosh kumar. Iam a fresher.

when Iam working on WEBI reports, I didn't understand the concept of drill..

even i made a report in WEBI  by taking the data source as universe.

so can anyone elaborate me clearly regarding the concept of drill in WEBI reports..

Thanks..

Kind Regards,

Santosh Kumar.