Question

What is the difference between |-3| and -3

Answer 1

|-3| means the absolute value of -3 the absolute value of -3 is 3
-3 is not the same as 3

Answer 2

The two lines in |-3| represent something called absolute value, which means that you interpret that value as its distance from 0. Distance is always positive, which means any number's absolute value is positive.
The difference is that |-3| is positive and -3 is not.
Hope that helped you.