A MEMO or Tag is an identifier assigned by an exchange to each user. Exchanges use one or a few addresses to receive ATOM from multiple users, so they need a MEMO to identify whose account to credit.
imToken is a decentralized wallet where each user has their own wallet address. You do not need to enter a MEMO when withdrawing ATOM from an exchange to imToken.
However, when transferring ATOM from imToken to an exchange, you must enter the MEMO provided by the exchange.