You learned in Math 9 and Math 10 that expressions with variables allow you to replace the variable with a number, and evaluate the result.
'Evaluate' means fill in the given number and work it out.
For example, evaluate x2 + 3x + 1 when x is 4
x2 + 3x + 1
= (4)2 + 3(4) + 1
However, there are some circumstances when you cannot work out the answer.
Here's an example of one of those:
Evaluate when x = -3
When you fill in -3 for x, the bottom of this expression becomes zero, since -3 + 3 = 0
For this expression, -3 is called an undefined value, or a restriction, or a non-permissible value.
It's the value of x that would make the bottom of a fraction equal to zero ... and division by zero is impossible.
So when you're asked to find the non-permissible values of an expression, you need to find the values of x that would make the denominator work out to zero. Those are the values that can't be used.