In 'Serious Engine', 3D world can be imagined as a set of entities (everything is an entity: sound, enemy, item, light, architecture, ...). Entities can be rendered as brushes or models. Models are objects compiled with 'Serious Modeler'. They are build from many polygons (enemies, trees, statues, ...). Models use level of detail (LOD) system for fast rendering. Brushes are entities that form world's architecture. They are usually big solid objects like a building or a mountain. Brush contains sectors, sector contains polygons. Each polygon has up to three layers of textures (picture or series of pictures saved in a specific format). Polygons between sectors are called portals (these terms are specific to engine, but are very important). Portals are used in dynamic visibility determination.
If you are creating a game from scratch, you have to create models, create entities (programming classes of entities), and build worlds. First you create some 3D models using Serious Modeler. After that, you code classes to use these models. Then you build a world using Serious Editor. Create textures from targa pictures and texturize your world. At the end you drop some entities into world and adjust their properties.
If you create custom levels, you build worlds using already existing entities and altering their properties.