A memo is a message attached to a transaction on the Hedera network.
Memo's are required by exchanges such as Coinbase and Binance when making a deposit.
When a user sends funds to an exchange, a deposit address and memo will be provided by the exchange. The address is where the funds go on the Hedera network and the memo acts as a routing number to ensure the funds are correctly associated with your exchange account.
When sending funds from an exchange to your personal wallet a memo is not technically required for the funds to reach you however some exchanges require the memo field to be anything except blank. In this instance you can enter anything as the memo for your personal records e.g 'withdrawal', 'abc123', 'I <3 HashPack' etc.
When sending funds from HashPack to HashPack a memo is not required unless you wish to include a message. This message will be visible on the Hedera network.