I have a python programming question

In this code we want to determine if a sentence consists of all letters and if so then it is a pangram.

Below is the correct code but I dont understand what the: if '' not in char, does? what is the ''?

def pangrams(s):

unique_letters = set(char for char in s.lower() if ' ' not in char)

if len(unique_letters) < 26: return 'not pangram'

return 'pangram