Whole heads of garlic are pickled in a solution of salt, vinegar, and sugar until the peels are soft enough to eat and the garlic pulp turns into a paste when pressed with a spoon. Pickled garlic (and pickled garlic juice) is an ingredient in quite a few Thai dishes, especially Central and Northern dishes.
Outside Thailand, pickled garlic comes in a glass jar as shown. It’s available at most grocery stores catering to Southeast Asian shoppers. They can also be purchased online.