A memo (also called a tag or comment) is an extra field attached to an address when sending crypto, by which an exchange decides which account to credit when the address is shared.
In plain words
Some networks and exchanges (for example TON, XRP, Stellar, Cosmos) use one shared address for many customers, and the difference between them is in the memo, tag or comment. If the field is left out or entered wrongly, the funds may reach the exchange but not your account: you will have to contact support, sometimes for a long time. So copy the address and the memo together from the deposit page and do not change them. On networks that need no memo (for example TRON or Ethereum) the field is left empty.
Example. You top up an exchange in TON: the page shows an address and a numeric MEMO. If you send without the MEMO, the exchange receives the coins but does not know whom to credit.