A mnemonic seed is a set of 12 or 24 words that represents a specific sequence of cryptographic keys which control your Dash balance. Since spending Dash involves signing creating transactions, losing or giving your seed to someone else is the same as losing or giving away your Dash. However, since the seed is only used when creating or restoring a wallet, it can be used to restore access to your Dash if your phone or laptop is stolen. This is an important advantage over the traditional cash in your wallet, for example, which is permanently lost if stolen.