Put the last two lessons together. Over/under billing equals billed to date minus earned revenue. If you have billed more than you have earned, you are over billed; if you have earned more than you have billed, you are under billed.
On the balance sheet, over billing is a liability (often called billings in excess of costs and estimated earnings): you have collected or invoiced for work you still owe. Under billing is an asset (costs and estimated earnings in excess of billings): work done that you have not billed yet.
Neither is automatically good or bad. Modest over billing is common and helps cash, but it is money you will have to earn by doing work, and if it is spent the back end of the job gets tight. Under billing can be harmless timing, but persistent or growing under billing is a warning: it can mean unbilled or unapproved change work, a slow billing process, or an estimate that is too low so percent complete is overstated.
Sample data: Riverside is under billed by about $2,778,781. A good WIP review asks why, job by job, every month.