Practice is a play mode in The Elder Scrolls: Legends. It allows the player to play matches against AI opponents of difficulties ranging from Novice, Adept, to Expert. This allows the player to become more familiar with the game without any stakes at hand, and also test out strategies for decks.
Initially, Adept and Expert are locked. You need to win a match in Novice to unlock Adept, and win one in Adept to unlock Expert. Upon winning, you will receive 5, 10, or 15 Soul Gems, depending on the difficulty. The amount of Soul Gems that can be earned through this mode is capped at 300 per day. However, this can be partially avoided by first earning 295 Soul Gems (most easily obtained by winning 19 Expert games and 1 Adept game) and then winning one more Expert game to obtain a total of 310 soul gems.
A list of all opponents and their decklists in each difficulty can be found here.