A .BIN file describes a three-dimensional model for MTM (and Hellbender, and CART, and MTM2, and...). The basic structure of a .BIN model consists of points in 3D space. These are usually called vertices (singular: vertex). A vertex is not visible in the game, but BINedit can display them. The following diagram shows a hypothetical arrangement for 4 vertices.
Three or four of these vertices can be used to form a visible surface by being designated as the corners of a three- or four-sided polygon. These polygons are known as faces.
A face has only one visible side. The face is visible from the side where the vertices are seen to be defined clockwise. This face will only be visible from your side of the screen, if it is defined as 1,2,4,3 or 2,4,3,1 or 4,3,1,2 or 3,1,2,4.
Reverse the order, and the face is only visible from within your monitor, and becomes invisible from your side of the screen.
Each face uses a .RAW file (the image format used for all TRI texture files) for the rendering of its surface. The texture is said to be "mapped" to the face.
Several faces can share the same vertices and textures (or part thereof).
When you have lots of these vertices forming lots of these faces, mapped to a couple of texture files, you get a complete model of a truck (or a spaceship, or an Indycar, or...).
As its none-too-subtle name suggests, BINedit is a utility for editing these .BIN models. It allows you to create, remove, modify and manipulate vertices and faces to build your own 3D masterpiece and then - best of all - play with it in the game!
And really, that's all you need to know, but if you're gagging for the technical details you can check out Oliver Pieper's notes on the MTM Bin File Format.