Numerous means existing in large numbers or in great quantity. When something is numerous, there are many of them.

If a teacher says there were numerous spelling mistakes in your essay, she's telling you there were a lot of them, more than a couple, perhaps five or ten or more. When a park has numerous trails, hikers can choose from many different paths. A scientist studying numerous species of beetles might be examining dozens or even hundreds of different types.

The word carries a more formal, measured tone than simply saying “many” or “lots of.” You might tell your friend you saw “lots of cool rocks” at the beach, but a geologist might write that “numerous geological samples” were collected. Both mean the same thing, but numerous sounds more official and precise.

Notice that numerous describes things you can count, even if you're not giving an exact number. You wouldn't say “numerous water” (water is uncountable), but you could say “much water” or “numerous puddles” or “numerous drops of rain.” When you encounter numerous in your reading, the author is emphasizing that whatever they're describing appears or exists in impressive quantities, whether it's numerous examples, numerous attempts, or numerous possibilities.