Deterministic wallet means that all of the keys/addresses that are used by the wallet are generated from the wallet's initial seed. (As opposed to the older classic style where 100 keys are pre-generated randomly, and more keys are randomly generated as needed.) The benefit of a deterministic wallet is that a single backup is enough to cover all of the funds that will ever be kept in the wallet. (In the classic wallet style, if you don't make a new backup regularly every 100 addresses, then funds in later addresses won't be accessible through the backup.)