The left hand signature indicates there are 4 beats to the Bar ( the sheet music is divided by vertical lines indicating the Bars) denoted by the top 4. and that the note used to count the beats is a Quarter Note ( explained later). This time signature is the most common in Western Music and is also as Common Time.

The middle signature indicates there are 3 beats to the bar each of a quarter note duration. Waltzes are a classic example of this time signature.

The right hand one indicates 2 beats to the bar, each of a quarternote duration.

Other examples are 6/8 8/12.

This symbol is found in sheet music immediately following the Time Signature