"Unfoot" in a community defined term that describes how you can make a blocker resume walking, by removing the land beneath the blocker's feet. The blocker will resume walking in the direction it was going prior to being assigned that skill. It is not an official term given by the developers, and other walkthrough authors have used different terms such as release. You can unfoot a blocker by using any combination of excavation skills that remove terrain beneath the blocker. The most common excavation skill for this application is the miner.
Some ports such as Lemmini allow you to assign a blocker to a miner at a specific point in the miner's stroke, and the blocker will instantly unfoot due to lack of terrain beneath his feet. This allows you to stop a miner without a builder.