The suffix -or attaches to the end of verbs to create nouns that name a person or thing that performs an action. When you add -or to a verb, you get the name for whoever or whatever does that action.

An actor is someone who acts. An inventor is someone who invents. A conductor conducts an orchestra or a train. A generator generates electricity. Notice how the pattern works: the base verb tells you what's being done, and -or tells you who or what is doing it.

Sometimes choosing between -or and its cousin -er (which works the same way) seems random. We say teacher and baker, but instructor and senator. There's no reliable rule for which one to use. You just have to learn them as you encounter new words.

When you see -or at the end of a word, you're almost always looking at the name for someone or something that does the action in the root verb. This makes -or one of the most useful tools in English for building new words and understanding unfamiliar ones.