This ABS() function returns absolute value of a number, for example, the absolute value of 5 is 5, and the absolute value of -5 is also 5. In Mathematics, absolute value can be think of as a distance from zero.

ABS function examples

SELECT ABS(-15) FROM DUAL; -- 15
SELECT ABS(-51.321) FROM DUAL; --51.321
SELECT ABS(-20+12) FROM DUAL; -- 8
SELECT ABS(-5*6) FROM DUAL; -- 30
SELECT ABS(12/-3) FROM DUAL; -- 4

References

1. ABS Function :- Oracle official docs
2. Wikipedia – Absolute value