Security seed is a very simple way to memorize or save your private key. The private key of a Wallet consists of 52 random characters between letters and numbers, so it is a complicated task to memorize or write down since if you fail in a single character you cannot restore your wallet.
With the seed is much easier because these 12 words "easy" to memorize are used in everyday language and the sum of all of them in the correct order is equivalent to your private key.
REMEMBER: It is very important to write down the 12 words in a safe place when creating a wallet. In the case of loss of your smartphone you will be able to restore the wallet and access your cryptocurrencies thanks to these 12 words.