What do the engine annotations 0.53/24 mean?

The score 0.53/21 means the chess engine evaluates the position after 6... Ng8-f6 at 0.53 pawns (or 53 centipawns) in White's favour after completing a search depth of 21 ply (or half-moves). It means White has a slight advantage.

Similarly -0.25/24 means the engine evaluates the position after 6... e7-e6 as 0.25 pawns (or 25 centipawns) in Black's favour after a search depth of 24 half-moves.