The numbers represent what we call FPS, this means Frames per second. A game's frames per second is a measured average of a series of tests. That test often is a timedemo, a recorded part of the game which is a 1:1 representation of the actual game(play). After forcing the same image quality settings this timedemo is then used for all graphics cards so that the actual measuring is as objective as can be for all graphics cards. If a card can only manage <30 FPS then the game is barely playable. With 30 FPS up-to roughly 40 FPS you'll be very able to play the game with perhaps a tiny stutter at certain graphically intensive parts.

When a graphics card is doing 60 FPS on average or higher then you can rest assured that the game will likely play extremely smoothly at every point in the game.