Whenever I ry to decimate my model, it says it is a manifold mesh and it won't decimate it. What is a manifold mesh and what do I need to do to decimate my model?

well that error is caused because Blender's decimator can work, you each face can only have 3-4 faces attached i think. I don't really remember if it's faces or vertex, or 3 to 5, but just a bad explanation till someone else explains it the way it really is.

A MANIFOLD is a manifold mesh with no boundary. All edges have exactly two incident triangles. All vertices have exactly one cycle of incident triangles. All triangles in the mesh are consistently oriented.

Sometimes 3d modeling is just plain hard work.
Maybe one should take care not to create a model like that.
Non manifold meshes are "impossible" in real life anyway, so if you model the correct way you wouln't get a non manifold mesh.