Complex sentence: Prior to the release of the "Source engine", the Editor used only brushes (blocks) called primitives sets of simpler primitives, they can be used in older games that are not based on the "Source engine".
Split into simpler sentences: Prior to the release of the "Source engine", the Editor used only brushes (blocks) called primitives. Sets of simpler primitives can be used in older games that are not based on the "Source engine".