The number in the green rectangle to the left of the station location name is the PlugScore. 

A station's PlugScore reflects the experience of its most recent users. When a PlugSharer reports a negative experience at a station, its score drops. When a PlugSharer reports a positive experience, the station's score rises. A neutral comment or check-in does not affect PlugScore.  

Tap on the station location to arrive at this screen:

If you tap on the green rectangle above the "Check In" button you will be directed to a description of PlugScore: