Your mnemonic seed is the 13 or 25 random words that make up your private wallet key. When you first created your wallet, you would have been prompted to write it down and store it safely.
This seed phrase is highly important because it is the only way to re-gain access if you are locked out of your wallet.
Unfortunately, if you have not followed those instructions, there is no way for us to recover it. It is important to note that the MyMonero servers do not store or ever have access to mnemonic seed phrases. Our software is designed this way to protect your privacy and keep your wallet information safe.
Make sure to back up your mnemonic seed and have access to a copy of it at all time.
If you lost it and are somehow locked out of your wallet, we are unable to provide you with your mnemonic seed.