Imagine you're in Germany with a headache, and you want to tell a colleague that you have to see a doctor today. Expressing ability, necessity, desire, or permission — "I can," "I must," "I want to" — is something every language does, but German handles it with a very specific mechanism that you'll use constantly, so it's worth understanding well from the start.
These words are called Modalverben (modal verbs), and they're called "incomplete" because they don't carry full meaning by themselves: a verb like müssen (must) doesn't tell you anything concrete until a second verb explains must do what exactly. That's why a modal verb almost always appears together with a second verb that carries the real action.
The mechanic that trips learners up is this: the modal verb is the one that conjugates for the subject — ich muss, du musst — and it takes the second position in the sentence. The main verb carrying the real meaning, like gehen (to go) or arbeiten (to work), never conjugates at all. It stays in its base form, the infinitive, and gets sent literally to the very end of the sentence, no matter how many words come between. So in Ich muss heute zum Arzt gehen, the words heute zum Arzt sit between muss and gehen, but gehen is still waiting for you at the end.
Many learners come from languages where the two verbs simply sit next to each other, with no concept of "sending" a verb to the end. That habit produces the most common mistake: writing Ich muss gehen heute zum Arzt instead of the correct Ich muss heute zum Arzt gehen. Remember: no matter how the sentence starts, the unconjugated main verb is always the last word you write or hear.