What does "BRep" mean in OpenCASCADE?

BRep in class and function names like BRepGProp means Boundary Representation. That means representing a geometrical object by its boundary, i.e. its outer shell. For example, a cylinder in boundary representationwould be represented by two circles (its end faces) and a cylindrical shell.

BRep ist the way STEP files represent their geometry data - in contrast to, for example, STL files, representing the geometry by a buch of triangles connected together. Boundary reprensentation allows exact representation of geometries whereas representing objects with triangles or other methods often involves a loss of precision, i.e. the original geometry will be slightly different to the geometry as stored in a STL file.