Net 30 means that in 30 days you can call and ask why you haven’t been paid. You’ll be told that since 30 days have passed the bill will be submitted to accounts payable. In a week or two AP will process the bill and set a payment date for the next scheduled payment date based on approval by management. When that payment date arrives (perhaps another 30 days) management will have some questions about the work performed and payment will be rescheduled after those concerns are addressed. When that is done the next payment date will be scheduled again, probably another 30 days, and when that day arrives you can call and ask where your money is and you’ll be told checks are cut on some upcoming date (don’t worry, it’s probably only around 15 more days now). The check will be cut but not sent out until you call again. Expect another 10 - 15 days for the check to actually be mailed. So maybe after 90-100 days you’ll get paid.
Okay, so it’s not always that bad. But if you’re going to be in business for yourself you should be prepared for payment to take much longer than the terms specify. Defaults are rare, late payments are common.