This document breaks down the use of Geometry Brushes in level creation.
Geometry Brushes are the most basic tool for level construction in Unreal. Conceptually, it is best to think of a Geometry Brush as filling in and carving out volumes of space in your level. Long ago, Geometry Brushes were used as the primary building block in level design. Now, however, that role has been passed on to Static Meshes, which are far more efficient. However, Geometry Brushes can still be useful in the early stages of a product for rapid prototyping of levels and objects, as well as for level construction by those who do not have access to 3D modeling tools. This document goes over the use of Geometry Brushes and how they can utilized in your levels.
In general, you can think of Geometry Brushes as a way to create basic shapes for use in your level design process, either as permanent fixtures or as something temporary to test with while your artists finish creating final meshes.