Explain the difference between |-3| and -3?

Absolute value means how many spaces away from zero a point is.

If a point is placed at |-17| then it means the point is either 17 spaces positive or negative.

Also, you might not be talking about graphing and in this case then the absolute value of anything is positive while a number alone is just that number.

They are opposites. since |-3| has the lines around it, this represents absolute value, which is not positive or negative but how far away the value is from 0. the plain -3 remains the same. |-3| is 3, because it is 3 units away from 0 on the number line.