Suppose in April, 2011 your company has issued 10000000 esop options to at EGM.
In May, 2012 you call a Board Meeting or any Committee Meeting and out of 10000000 option you granted 2000000 options to eligible employee as decided in Board/Committee Meeting.
Suppose Mr. X received 100000 option on Board Meeting then the date of Board Meeting will be called as Grant Date.
In ESOP Scheme, you will mentioned the period for which, the Scheme will be operational, like 5 years or 7 years.
The period will be called vesting period. So, Mr. X will be eligible to exercise their 100000 options in vesting period of 5 or 7 years.