What is a tag / memo and is it always required?

The TAG/MEMO is a unique identifier assigned to each account that allows centralized cryptocurrency exchanges to identify a deposit and credit the appropriate account. By forgetting the MEMO, your transfer may not arrive at the destination until it's retrieved. Binance.US requires a tag/memo.

I.e. Trust wallet is a decentralized wallet and it does not require a Memo or Destination Tag to send tokens. You only need to input the wallet address correctly. Please read more here.