An exchange keeps one shared address for everyone and sorts incoming funds by their markers. Without a marker the transfer arrives at the shared address and matches no account: the money is on the network, but not in your balance.
What to do if you forgot
Contact the recipient's support with the transaction hash. Such payments are often found and credited by hand, but that takes days and is not guaranteed.
How not to forget
The marker is issued together with the address and always sits right next to it. If a service shows a memo, it is mandatory, even when the field in your wallet is labelled optional. For a withdrawal to a personal wallet, no marker is usually needed at all.
Where the marker appears
On XRP it is called a destination tag, on Stellar and TON a memo, on Cosmos a memo, on EOS a memo. The name varies, the meaning is one: without it the recipient cannot tell whose payment it is.
On TRON, Ethereum and BNB Smart Chain there is no marker at all, since each customer gets their own address. That is why the habit of "no memo needed" carries over from one network to another and leads to losses.
What a marker is not
A payment reference in the banking sense. A marker is not text for a human but a service number: typing "payment for USDT" into that field is pointless and sometimes harmful.
It is copied the same way as an address, in full and without edits.