Obtaining source material from already built/compiled material.
In Half-Life, generally refers to taking a .bsp apart into a .map or a similar editable format. Usually goes poorly, with much of the .bsp brushes lost.
Models (.mdl files) can also be decompiled, generally without issue.
Publishing, without permission of the original maker, a work that you have decompiled and changed is generally considered stealing by other modders, even if you only give it to a few friends. Decompiling can be helpful in finding out how someone did something, but why not just ask?