How to organize textures?

I have a big project that has 300 textures and sketchfab only support 100 textures when I upload to their cloud, so I need to reorganise them, making the same textures into one and etc, are there a plugin that help me better with this?

Other question is: How to join every mesh like sketchfab says to do here: what they mean by mesh? is it a model?

Mesh is the actual geometry of the model, disregarding all groups and components. Groups and components is how different meshes are organized in SketchUp. Other software might do things differently.

If you have those many textures and they are really unique (not similar or duplicated materials and textures), the best way is to use blender. Duplicate the model, remove textures from duplicate, uv unwrap it and bake the textures of the original into the unwrapped version.

If this seems complex it’s because it is, but it’s the best way to do something as complex as it seems by your description.

Otherwise, do it manually, if you think it might be faster.

what the bake process does?

What @MikeWayzovski quoted. You should find some nice examples on YouTube and just follow them.