This is the key to understanding this concept. Unfortunately, it can be pretty confusing. The gist of the matter is that warmer air can hold more water than cooler air. When cooler air starts to hold too much water, that water vapor coalesces into general water, which has to collect somewhere.
Essentially, that means a crawl space sitting at 65°F can hold substantially less water vapor than an outdoor area at 95°F. If the warmer, wetter air comes into contact with the cooler, drier air, that water vapor will collect somewhere in your crawl space.
You can do calculations to determine exactly when the water vapor will start to condense, but for these purposes, you just need to know that it will. When the condensation forms, you’re going to start having moisture problems.