Most people who collect unemployment benefits are completely out of work. However, you may also be eligible for benefits if you are underemployed: working only part time or occasionally, even though you'd like to be working more. These are called partial unemployment benefits.
State law determines your eligibility for partial unemployment benefits. How much you can collect is also set by state law, although states typically pay the difference between what you would collect if you were fully unemployed and what you are actually earning, less a small allowance. (Most states don't count a small portion of what you earn, to provide an incentive for applicants to take work if it's available.)