So...less and fewer mean the same thing, right? But because English likes to be confusing, we use therm in different contexts. In general, we use "less" for non-count nouns (i.e., things we can't easily count like water, grass, sugar, flour sand, etc.) and "fewer" for count nouns (i.e., things we can easily count, like books or chairs or apples).
Here is a beach with lobsters and sand.
Here is another beach. It has fewer lobsters and less sand: