What is an IOTA Wallet Seed?
To put it simply:
Your IOTA seed is like your private key (similar to bitcoin and ethereum). It is 81 Characters in length. With it you can open your wallet on any computer and generate addresses on which to receive IOTA.
Keeping your IOTA Seed Secure
Unlike other crypto currencies, the IOTA wallet does not store your IOTA. Your IOTA is stored on The Tangle and your seed is needed to access it, verify ownership and display the amount in the wallet. It is Vital that you keep your seed secure. Once someone else has access to your seed and thus your IOTA they can me moved out of your wallet. This is an irreversible transaction and you will lose your investment.