The slip/skid indicator is the small bar under the roll pointer. The image below should clarify that.
Image from flightlearnings.com
The Slip/Skid Indicator (sometimes called turn-and-bank indicator) moves left and right relative to the roll pointer in proportion to lateral acceleration and helps the pilot correct for any deviations in a turn.
In a level flight, the indicator is centered. Assume that the pilot is executing a left turn (the roll pointer would've moved left)- if the indicator is in center, it means that the turn is coordinated i.e. the aircraft is not slipping or skidding.
If the indicator (bar) is moving right, it shows that the aircraft is skidding. Movement in the opposite side (i.e. towards the left, same direction of turn) indicates that the aircraft is slipping. This enables the pilot to take corrective action. This is one of the necessary flight instruments.