a 3d structure that makes up a 3d model in a video game
to create a mesh, you start with a group of vertices, then draw edges between those vertices. now you have faces, and faces make up the surfaces of your 3d object
you can also warp textures onto the surfaces of your mesh using UV coordinates to give the 3d model more detail (color, shadow, shininess, surface bumps, glow, transparency)
meshes can be made in 3d modeling software such as blender. you an make a mesh out of any polygon, but by the time it reaches the gpu, it is always triangles