What are meshes? How do you turn them into clothes?

You don't turn a mesh into clothing, but the two do work interchangably.

Think of a mesh like a clothing pattern. How do you get boot leg jeans? The bottom part of the jean is flared so it fits around boots. Skinny leg jeans, there is no flair. That's what a mesh is: defining what the clothing looks like, the style of it.

You have to use special programs to create meshes, and you can find some decent tutorials at http://www.modthesims2.com under "create".

Your best bet would be to learn how to make the clothes. While it doesn't always work out well, you could take a mesh with a t-shirt and boot leg jeans, and a mesh with a t-shirt and skinny jeans, and use the same skin on both. The skin (clothing) wraps around the mesh (pattern) so the game knows how to style it. So use meshes that people have already created and permit you to use, learn to make skins, then focus on mesh making.

There is another program the teach you how to make a new clothes for them.

there are many ways - fashion magazines explain all the details.
at times with innovative ways you can make millions by suggesting clothes for models - they are made for one time use - not convetional one