Why do some coins need a memo/tag?
On chains like XRP, XLM, EOS, ATOM and HBAR, an exchange gives lots of users the same shared deposit address. The address alone can't tell whose money a deposit is — the memo/tag (sometimes called a note or a label) is the "apartment number" that credits it to your account. Leave it off or get it wrong, and the money lands at the exchange's shared address with nobody's name on it, and it takes an extra process to have any chance of getting it back.
Coins like BTC, ETH and TRC20-USDT, by contrast, give each user their own unique address, so the address itself already points to you — there's no memo field, and none is needed. If a coin's deposit page has no memo/tag box at all, that's your answer: nothing to fill.
What if you forgot to add it?
If you deposited to Binance but forgot the memo/tag, some coins let you file a self-service "missing tag recovery" request (whether it's supported depends on the coin, there's usually a fee, and recovery is not guaranteed — it comes down to the outcome of Binance's own self-service process). For the steps and the fine print, read what to do if you didn't add a memo/tag. Before your next deposit, use this tool to check whether you need one.
It only tells you whether a given coin/network usually needs a memo. It doesn't check the live status of your deposit, it can't recover coins that are already stranded, and it can't guarantee its read always matches Binance's current setup. Go by the field Binance's deposit page actually shows, and always send a small test first.