In short, O(1) means that it takes a constant time, like 14 nanoseconds, or three minutes no matter the amount of data in the set.

def add_items(n):
    return n + n + n
 
print add_items(10)

O(n) means it takes an amount of time linear with the size of the set, so a set twice the size will take twice the time. You probably don't want to put a million objects into one of these.
This is always the best scenario possible to take and is represented as constant in the initial image.