IDCW stands for Income Distribution cum Capital Withdrawal, and it's a mutual fund option where the fund periodically pays out a portion of its gains to you in cash, instead of reinvesting them. Each time a payout is made, the fund's NAV drops by roughly the same amount that was paid out, since that money has literally left the fund and gone into your bank account.
This option can suit investors who want a regular stream of cash from their investment, such as retirees supplementing their income. However, because payouts reduce the NAV and interrupt compounding, and are also taxable as income in the year received, IDCW generally results in a smaller overall corpus over the long run compared to the growth option. It was previously known as the Dividend option under a different name.