The memo field is a reference field for you to be used to add any information you may want. Think of it as the "concept" field you have when sending a bank transfer.
⚠ The memo field is a mandatory reference to be added when you're sending your funds to a CEX (Centralized exchange service). ⚠
And what's a CEX? It's an online trading platform that matches buyers and sellers via an order book. They are popular and usually make you go through some sort of customer identification (KYC). Coinbase, Kraken, and Binance are CEXs.
If by mistake, you've sent your funds to a CEX and you forgot to add a Memo or Reference, you'll need to reach out to the relevant CEX support team to seek assistance.