What's that thing at the bottom of the fork steerer? You know that slanted metal thing on the bottom of the fork steerer that can slide around? Like, the diagonal cut thing at the end that you insert into the hole when you assemble a bicycle? What is that part called, and what is its function?

I think by _steerer_ you mean a _quill type stem_

!quill stems

The diagonal cut part is a wedge. It's pulled up by a long bolt that runs down the stem and locks the stem in the fork steerer tube.