Some of them create a new meaning (becomes a phrasal verb) - in other words, when put together the two or more words act as a completely different verb. For example, “to work out” means to exercise (typically with the goal to be fit or develop certain skills or physique; more intentional/goal-directed than the meaning of “exercise”). This is completely different from “to work.” Other times, adding “out” does not change the meaning as drastically; for example “help” and “help out” mean close to the same thing, although the usage is somewhat different. “Help” can be modified by a following verb, as in “help me make dinner,” whereas “help out” is more general and stands alone, as in “I came to help you out” (you cannot say, for example, “I came to help you out make dinner.”) As Clara (Blue) said, the phrasal verbs simply must be learned because they are essentially a different word. Hope this helps! :)

Thank you @Clara(Blue) :)

They're called phrasal verbs. Once you add that second part, the meaning changes. A phrasal verb is a verb that is made up of a main verb together with an adverb or a preposition, or both. English has hundred of them, so you'll have to study them progressively.