When you create an EOS deposit address on your Kraken account, a unique "Memo" is also generated. This Memo is an extra piece of information that can be included when sending an EOS transaction.
On Kraken, the Memo is used to determine what account a given transaction should be assigned and credited to. This is because all Kraken accounts use a single EOS deposit address:
The Memo on Kraken is a unique numerical message and has to be included with all EOS deposits to your Kraken account. An example Memo is:
Always ensure to include the correct Memo before sending your EOS deposit. Leaving out this information will lead to significant delays in crediting the transaction to your account, and can be irretrievable in certain cases.
Note: EOS (as well as XLM, XRP, and XMR) cannot be sent between two Kraken accounts, and any such transaction will result in the transaction failing with a "Failure"-status.
If you forgot the memo submit a support ticket here.