Refer to figure.
The turn indicator is a gyroscopic instrument designed to indicate an aircrafts rate and direction of turn, the turn coordinator work in a similar way. Both instruments usually incorporate a balance indicator (balance ball) to enable pilots to fly balanced turns. The balance or “slip” indicator comprises a curved, liquid filled, tube that contains a ball which moves in a similar way to a pendulum. It gives an indication of the state of balance of an aircraft in a turn or straight and level flight.

In level flight gravity will keep the ball in the middle, when turning centrifugal force proportional to the TAS and rate of turn also acts on the ball. If the lift is equal to the resultant of the aircraft weight and centrifugal force then the turn is balanced and the ball will stay central. A properly balanced turn is where the angle of bank is correct for the TAS and rate of turn, too much bank (not enough rudder) causes the aircraft to slip into the turn and too little bank (too much rudder) causes the aircraft to skid, when a turn is balanced the ball will be in the middle.

• Slip - If the ball is off centre in the same direction as the turn, the aircraft is slipping. Increasing rudder deflection in the direction of the turn (step on the ball) or reducing the bank angle will bring the ball back to the middle. This is a little similar to understeer in a car, as the aircraft is pointing out of the turn.
• Skid - If the ball is off centre in the opposite direction to the turn, the aircraft is skidding. Increasing rudder deflection in the opposite direction to the turn(step on the ball) or increasing the bank angle will bring the ball back to the middle. This is like oversteer or drift in a car, the aircraft is pointing into the turn.

Note: Phrases to help you remember the difference between slip and skid are: 'SLIP IN' and 'SKID OUT'
If the ball's on the INside of the turn you're SLIPping ( Slip In), if it's on the OUTside of a turn you're SKIDing (Skid Out)!

As the turn is to the left and the balance ball is also to the left, then we are "slipping in". It is a slipping turn. To coordinate this turn, apply more left rudder or reduce bank angle.