Those lines stand for the absolute value, or how far away it is from zero.
If you draw a number line from -3 to 3, and plot the points x = 2 and x = -2, what similarities do you see?
Although one is two units to the right of 0 and one is two units to the left of 0, both x = 2 and x = -2 are two units away from zero. Therefore, the absolute value of both 2 and -2 is 2.