What does 25t mean when talking about a bike sprocket?

When talking about a bike sprocket, "25t" means that the sprocket has 25 teeth. The number of teeth on a sprocket affects the gear ratio of the bike, which in turn affects how hard it is to pedal and how fast the bike will go. A larger number of teeth on the sprocket makes it harder to pedal but will give you a higher top speed, while a smaller number of teeth makes it easier to pedal but will give you a lower top speed.