The HELP menu in Tally describes EFFECTIVE DATE. Refer topic Voucher-types which provides explanation for Effective Date. The same is given below for your reference.
Use EFFECTIVE dates for vouchers?
Select Yes if you want to enter effective dates for vouchers. You would opt for this if you have instances where a transaction under consideration for overdue/ageing analysis is recorded currently but will take effect from another date. If effective date is entered, the overdue/ageing will be considered from the effective date and not from voucher date.