what does if not mean in python code example

Example 1: python if not true

a = False
if not a:
  #Does this

a = True
if not a:
  #Doesn't do this

Example 2: is not in python

li = [1,2,'a','b']
if 'hello' not in li:
    print('hello is not in the list')

Example 3: if not python

a = False
if not a:
    yourFunction()