Margin Required — shown as Required on the order window, next to your Available margin — is the amount that gets blocked from your available margin if you place that specific order. It depends on the product you've selected — Delivery, Intraday, MTF, or T+5 — and the leverage applicable to that product.
Why does it change when I switch the product?
Each product tab on the order window — Intraday, MTF, T+5, Delivery — shows its own leverage multiplier and max quantity, and blocks margin differently. Delivery generally requires close to the full order value (1.0X), while Intraday, MTF, and T+5 use leverage, so the Required amount is a fraction of the order value. Switching the product changes the Required amount accordingly. See Which product should I choose — Intraday, Delivery, MTF or T+5? and, for Intraday specifically, How much leverage do I get for Intraday?
What if I don't have enough margin?
If your Available margin is less than the Required amount for that order, the order gets rejected. See What do ledger balance, withdrawable balance and available margin mean? for what counts toward your available margin, and Why do orders get rejected? for other rejection reasons.