"de" mean "of". "Beaucoup de" means "a lot of" We put "de" with most adverb of quantity (un peu de, beaucoup de, assez de, trop de) when you want to state the quantity of something . We also put it in negative sentences "pas de". Don't focus on the meaning of singles words, sometimes, it's better to see them as a pair or a trio to understand their meaning.