When you grant options to team members, those options come from a pool of reserved shares set aside for an option plan (also known as an ESOP or a Share Incentive Plan).

The unissued option pool is the number of reserved shares that have yet to be assigned as option grants for team members. For example, if you created an option plan with 150,000 reserved shares and then granted 50,000 options to team members, your unissued option pool would be 100,000.