A mesh is the structure of an object that tells the browser how much space it takes up and what shape it is. A texture is the colour or pattern that goes on the mesh so you can see it. For example if your avatar is wearing a plaid skirt, the shape of the skirt, how short or long it is, where it flares out, whether or not it has pleats are all part of the mesh. The plaid pattern is the texture.
Clothing design in the Legacy system only allowed for texture editing because the meshes were built into the client. This feature is still available in RLC 2.0, so if you'd like to simply re texture existing RLC 2.0 meshes you are welcome to do so.
Mesh design is a more complicated process that requires a mesh creator to be proficient with 3D modelling. The system for mesh creation is not yet available for use, but when it is available, users will be able to create all kinds of custom clothing shapes (different forms of skirts, tops, dresses, shoes, etc.). These custom items then will be added to the list of meshes available for texture designers for re texturing. Mesh creators will have an option to set prices on their creations.