To stipulate something means to demand that it be part of an agreement. So when you make a contract or deal, you can stipulate that a certain condition must be met.
Anytime you draw up a legal agreement, you can stipulate a requirement that has to be met for that agreement to be complete. This stipulation might put some sort of limit on the agreement. For example, if you run a fencing company and offer a sale, you can stipulate that to get the sale price, the fence must be ordered by a certain date. Your customer, in turn, might stipulate that the work must be finished before the ground freezes.