By now you know that Spanish has “masculine” and “feminine” nouns. Masculine nouns take el and los (el libro/los libros) while feminine nouns take la and las (la mochila/las mochilas). So, what is masculine about a book and what is feminine about a backpack? Absolutely nothing. These labels—masculine and feminine—are artifacts of the way ancients talked about language. Did you know that other languages have three or more genders? These categories aren’t really genders, then, just groups of words. They could just as easily be called A words, B words, and C words, or Group 1, Group 2, and Group 3. Unfortunately, grammarians got stuck with the term “gender” and there you are!
By the way, your tendency in learning Spanish will be to “default” to masculine and overuse el and los, using these articles where la and las are required, and to also overuse masculine endings on adjectives. This is natural and something we see in second language research. The only way to get gender under control is to constantly hear and see words in context in which the “gender” is easy to map onto the word.