A turn is best represented by a piece of string wrapped around an object. The string may be made up of several threads to form one piece of string. This is identical to how you would find a turn in a brushless motor. Many small gauge strands of wire placed in parallel to represent a heavier gauge wire. The wire is then wrapped around the stator tooth through the stator slots as identified in the thumbnail image.
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Does turn count effect performance?
Short answer is yes it does. The amount of turns that exist on a brushless motor is directly related to the kv of the motor. As the amount of turns on a motor increases, the kv of the motor decreases.
An ideal motor for maximum performance would have the smallest internal resistance. If you compare two motors of equal kv, the motor with fewer turns (in general) would be best. Internal resistance only increases when you increase the amount of turns in the same physical size motor.
Do you need to use the turn count for motor selection?
No, using the methods described in previous videos about brushless motors identifies the best method to selecting a motor. To summarize, you need to know the physcial size of the motor to suit your application and the kv value required to suit your application. There is little information gained from knowing the turn count of a brushless motor.