Stamps are snippets of wikitext that can be inserted into a tiddler at the click of a button, for use when transcluding some procedure or tiddler doesn't do the entire job. For instance, perhaps what you need to insert repeatedly actually is a procedure call, but it has a bunch of parameters which you can't remember well and you'd like to be able to insert a version with useful placeholder text.
Stamps are tiddlers (what else?) tagged $:/tags/TextEditor/Snippet. Such tiddlers appear on the stamp button on the edit toolbar under the name in their caption field, and clicking them inserts the text of the relevant snippet into the editor.
It's also possible to make a stamp take some selected text and add a prefix or suffix to it, rather than just inserting a chunk of new text. For this and more, see the documentation on stamps.
Exercises
Create a snippet Link to Wikipedia article that inserts a call to the wikipediaLink global procedure. Specifically, it should use the prefix/suffix feature mentioned in the previous paragraph so that if you highlight Aardvark, you get <<wikipediaLink "Aardvark" linktext>>.