Base monsters (ベースモンスター Bēsu Monsutā) are required for the Composition Summon of a Composition Monster. When the Level of the Base monster matches the Quality of the intended Composition Monster, and you have the proper monsters in your Graveyard, you can Summon the Composition Monster.
Much like Tuner Monsters, Dark Tuner Monsters, and Pendulum Monsters, Base monsters are primarily a secondary Type of Effect Monsters, but they can also be a secondary Type of Normal Monsters. Like the first of these two, there are cards that can treat non-Base monsters as Bases, including themselves, meaning Synergizing, like Tuning and Dark Tuning, is NOT considered an effect, and cannot be negated with cards such as " ".
While Tuners and Dark Tuners prefer either a low or high range of Levels (respectively), Base monsters run the whole gamut of Levels from 1 to 12, due to having to make their Levels match their Compositions' Qualities rather than add or subtract.
Some Composition Monsters require specific Base monsters to be Composition Summoned, but due to the Level-matching mechanic, Composition Monsters that list specific Bases also state "(must be Level X)" right after them, with X being the same Level as the Composition's Quality.