It refers to the grip size and affects comfort & control. If you are between two sizes, choose the smaller one, as it can be easily adjusted with an overgrip.
Lightweight rackets are suitable for children and beginners, while heavier ones (>290gr) are suitable for more experienced players, as they produce more power with the proper technique.
Rackets with larger head size generate more power and provide a larger sweet spot. Advanced players choose rackets with smaller head size that provide better control and generate more power.
The length of an adult tennis racket is usually about 68.58 cm (27"). Rackets with greater length require a slightly greater distance from the ball and provide more power in shots, but lack flexibility and maneuverability.