A person’s fear can keep them from doing what they want to do. For example, a fear of heights may keep someone from a trip up to the top of Seattle’s Space Needle or a fear of drowning may keep someone from swimming on the hottest day of the year.
A subcontractor may want to buy an expensive piece of equipment but can’t afford it or wants to bid a big job yet can’t because it requires a level of bonding he can’t provide. These are all examples of a constraint, which is defined as “something that limits or restricts someone or something.”