A motion of no confidence is a governance action declaring that the community no longer trusts the current Constitutional Committee. It is voted on by SPOs and DReps only. The committee itself does not vote, since the action challenges its legitimacy. If ratified, the committee is dissolved and a new one must be installed before normal governance can resume in full.
The same idea also exists as a standing delegation choice: Always No Confidence is one of the two predefined options an ada holder can delegate to instead of a DRep. Stake delegated this way automatically counts in favor of any open motion of no confidence and against keeping the current committee.