The absolute value of a number is its distance from zero on the number line, without considering its sign. In other words, it's the value of a number as if it were positive.
In the case of -1/2, the absolute value is 0.5 because that's how far -1/2 is from zero on the number line. When you take the absolute value, you ignore the negative sign and simply consider the numerical value, which is 1/2. So, the absolute value of -1/2 is 1/2, which is equivalent to 0.5.