The verb sein means "to be," and it is the single most important verb in German, because you will use it in almost the first sentence you ever say: introducing yourself, describing your state, naming your city or job or nationality. Without this verb, you cannot build a single introductory sentence in German, no matter how simple.
Here is the first thing to get used to. Many learners come from languages where you can drop the linking verb entirely — you just place the subject next to the description, and "to be" is understood. German never allows this. A form of sein must appear in every sentence of this type, with no exceptions. "I am a student" becomes Ich bin Student, where bin is a real word that can never be silently left out, however obvious the meaning seems.
sein is also completely irregular, so its six forms must be memorized like a single vocabulary item: ich bin (I am), du bist (you are, informal), er/sie/es ist (he/she/it is), wir sind (we are), ihr seid (you all are, informal plural), sie/Sie sind (they are / you are, formal). Note that capitalized Sie is used with strangers and older people, while du is reserved for friends, family, and children.
Word order follows a golden rule to lock in now: the verb always sits in second position in a statement, no matter what comes before it. Even if you open with a time word like heute (today), the verb stays second and the subject shifts to third place: Heute bin ich in München. In a question with a question word like Wie (how), Woher (where from), or Wo (where), the order is: question word, then the verb immediately, then the subject — Woher kommst du? The verb comes right after the question word, not after the subject.
The most common mistake learners make is dropping sein entirely and translating word-for-word from a language that doesn't need it, saying Ich Student instead of Ich bin Student because the meaning "feels" clear without it. Remember: any German sentence linking two nouns, or describing a state or location, needs a form of sein — always.