By working backward from all checkmate positions, tablebases determine with absolute certainty whether any position with few pieces is a win, loss, or draw, and exactly how many moves optimal play takes. They have overturned old human assessments and revealed astonishing wins that take well over a hundred moves of flawless play.
For practical players, tablebases are mainly a study and analysis tool: they confirm whether a tricky endgame is objectively winning and demonstrate the precise technique. Online engines consult them automatically, which is why analysis of low-piece endgames is effectively perfect.