The opposite of a number is derived when the value moves in the opposite direction on the number line. Hence the opposite of 10 on the number line is -10 because you simply move 10 units to the left of the number line. So to put it simply, the opposite of a number is the same number with a negative value. However the absolute value of a number is the positive value of that same number. Hence when a number is expressed in its absolute value (usually written as |x|, then whether its positive or negative, its value would be expressed as a positive. Which means for example, the absolute value of -10 is 10, (or | -10 | = 10).
The opposite of a number and its absolute value is always the same, because the value of any number in its absolute form is always positive.
Note that two numbers are opposites if they have the same absolute value.