Avg. year length is your average year length - in the case of most calendars, this will inform you how many days there are in an average year. This is affected by how many months you have, and how many leap days those months have. This is automatically calculated when you make any changes to your months or leap days.
Avg. month length is how your average month length. This simply adds up how many days there are in your months, and then divides that number by how many months there are in your calendar. This could help inform you how long your moon cycles should be should you want to fit one entire cycle (full moon to full moon) into one month.