AABB stands for “Axis-Aligned-Bounding-Box”. In 2D there is Rect2, which is only a point and a size (a pair of Vector2). AABB is the 3D counterpart, a point and a size as a pair of Vector3.
It’s not a collision shape, it’s only a super-basic math structure representing a box in space.
Doc: AABB — Godot Engine (3.1) documentation in English
AABBs are used in variables when you need to get the bounds of an object in space, because it’s more handy than having to carry two vector variables. For example, they are used as value type when getting the bounds of a VisualInstance node: VisualInstance — Godot Engine (3.1) documentation in English