Collisions are things that allow you to well... go on things. Image if you couldn't go on the rock-bridge in Fluorite Plains.
Oh, and of course! The saving part. When you're done loading your mesh, go to the 'Object' tab. Where it says 'Name', and 'Group', you can type in a name for the mesh (anything works), and what group you want it to be in (MyTrees, MyRocks, etc). If you typed in 'Jam' for the group part, it would make a new group. When you're happy with the mesh name, and group, simply click 'Save'.