Hi Megha,
Base entitlement period means system will consider the base entitl. amount ( in your sample scenario 15 days) in this period. So 15 days with base period 'calendar year" means EE's entitl. is 15 days for a year.
Accrual period means how system will assign base entitl. to EE. If you set it monthly system will give quota to EE ( I mean will put the amount in IT2006) each month. So employee can use his quota each month after system assigned him.
If you set your base entitl. 15 days for a year period and set accrual period monthly this means system will give 15 / 12 = 1.25 days quota per month to EE.
Max. Entitlement as is clear from the name means system won't generate more than this amount. Press F1 on to see a description for it.
Regards,
Omid