What does the expression "take it for granted" mean?

It indicates the statement following is a given and is true.

x + y =5. Take it for granted that x = 3

3 + y = 5

y = 2

In this case granted = given. Take it as a given. In other words the situation is plain enough as to need no argument.

" Assume that it will come to you " For example : I take it for granted that my father will send me money each month without fail

Be lazy because you assume everything will work out for you.