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.