When you talk about more than one thing — one book versus two or three books — you need a way to mark the plural on a noun. Some languages rely on a fairly regular plural pattern (a fixed ending), while others also have an irregular type where the word changes shape internally with no single predictable rule, so each form simply has to be learned individually. German plurals work much like that second, irregular type: there is no single ending that applies to every noun. Instead there are five main patterns, and no reliable rule tells you in advance which one a given noun will follow.
The five patterns: adding -e, as in der Tag → die Tage; adding -(e)n, the most common pattern with feminine nouns, as in die Frau → die Frauen; adding -er with an umlaut change on the internal vowel (a/o/u becomes ä/ö/ü), as in das Buch → die Bücher; adding -s for words borrowed from other languages, as in das Auto → die Autos; and sometimes no ending at all, only an umlaut change, as in der Vater → die Väter. This variety can feel overwhelming, but there's a real payoff: whatever the noun's gender in the singular — der, die, or das — the plural always takes the same article, die, with no exceptions. der Mann becomes die Männer, die Frau becomes die Frauen, das Kind becomes die Kinder. One article for every plural noun, which simplifies things a lot once you get past memorizing the pattern itself.
German flattens the article completely in the plural and leaves only a trace of the noun's original gender in the ending itself — feminine nouns ending in -in double the n in the plural: die Lehrerin → die Lehrerinnen. Because the plural can't be guessed with confidence from the noun's shape alone, the best strategy is to learn each noun's plural the moment you learn the noun, rather than trying to derive it later from a rule.
A common mistake is assuming the plural pattern can be predicted from the noun's gender (assuming every der-noun takes -e and every das-noun takes -er, for instance) — this isn't reliably true — or forgetting to switch the article to die and mistakenly keeping der or das with a plural noun.