PSF stands for: Pounds Per Square Foot. It basically takes into account the weight that a structure may endure per square foot. So it is not how much it IS under, but how much it COULD be under. But why is that important?
There are a lot of variables in any structure, and weight load is not always carried evenly. For instance, you may have a beautiful deck, so you invite 10 of your friends over. Let's say each of those friends weighs an average of 200 lbs. You all gather together in one small area of your deck for a group photo - now that one small area is carrying 2,000 lbs, including it's own weight.
In addition to that we have furniture, snow load, and other factors. So having the right spans and the right sized framing is critical for safety of people and for the building/structure itself.
In reality, these calculations are not hard to do. If you are starting on a project and need calculations, try an online calculator like this one. Either way, careful planning before any project is very important. So make sure you know your PSF and framing spans beforehand.