A seed is used as the source to generate all the private keys and addresses in your wallet. So if your computer crashes, you can regenerate all of your keys and access your bitcoins again.
All your addresses are derived from your seed.
The seed is essentially a very large random number that nobody in the world has generated before (or will generate in the future). This number is then converted in to a 12 to 24 word “mnemonic” to make it easier to store, but it just represents a number at the end of the day.
Now here's the important part: Keep your seed secure and do not show it to anyone.
If you lose your seed, you lose your ability to recover your bitcoins.
If someone sees your seed, they can use it to steal your bitcoins.
Do not take a screenshot of your seed, save it to a text file, or store it in the cloud. If your seed is exposed to the Internet, it’s at risk of being found by any malware that might make it in to your computer.
The simple and secure way to store your seed is to write it on paper.