A bundle is a collection of macros, commands, snippets, drag commands, templates, preferences, and language grammars.
For more info see the manual entry about bundles.
Bundles are kept on GitHub. Previously bundles were stored in a subversion repository and the guide linked below has not been updated.
If you have written a bundle that may have public interest you can write
textmate.org to ask for it to be added to the index, but be sure to first have read the style guide.
Other pages of interest: