Abstain appears in Cardano governance in two places. As a vote, it lets a DRep or other voter formally record participation on a governance action without taking a position: abstained stake is excluded when the Yes share is computed, so it neither helps nor blocks ratification. This differs from simply not voting, which is also outside the Yes tally but shows no engagement with the action at all.
As a delegation choice, Always Abstain is one of the two predefined options every ada holder can pick instead of a specific DRep. Stake delegated this way is treated as abstaining on every action automatically, keeping it out of all tallies while still satisfying the requirement to have a governance delegation set.