Why is there a quotation mark on the operators like 'mod'?

I am new to Haskell and I probably have some really dumb questions (so sorry about that).

I was reading some tutorials for starters and noticed that there are expressions like this:

removeNonUppercase st = [ c | c <- st, c `elem` ['A'..'Z']]

ghci> [ x | x <- [50..100], x `mod` 7 == 3]

I know what mod and elem means but why do they have a quotation mark on them?

Huge thanks!