-sion is a suffix that turns verbs into nouns, usually showing the action or result of doing something. When you expand something, you create an expansion. When you confuse someone, you cause confusion. When you divide numbers, you're doing division.

This little ending appears in many words once you start noticing it. An explosion is what happens when something explodes. A decision is what you make when you decide. Your vision is your ability to see. A collision occurs when two things collide.

Sometimes the verb form looks quite different from the noun: comprehend becomes comprehension, and permit becomes permission.

Notice that -sion often pairs with -tion, which does similar work. Sometimes which ending a word takes depends on the letters before it: after an “s” or “ss” you often get -sion (as in discussion from discuss), while many other words take -tion (like action from act). Both endings create nouns that name actions, processes, or results, giving you a way to talk about the doing of things as things themselves.