Class levels are not about the expertise or otherwise of you, the yogi. Rather they give you an indication of what to expect in that class. Note that they are based on physical demands on the body, your powers of concentration and skills in letting go of the monkey mind might be more challenged at Level 1.
Level 1 is a lower impact class, so most appropriate when energy is low. It may have greater emphasis on technique or on relaxation/restorative.
Level 2 raises the intensity of the class for a little more movement.
Level 3 is the most physically demanding, and may include challenging poses, higher heart-rate work, longer holds in standing poses to work the muscles.
You will also find combinations of these to give you a graduated indication of what to expect. Choose the
class that feels best to you today.