Opposite numbers are numbers that are the same distance from zero on a number line, but have opposite signs (positive vs. negative).
For example, on this number line, -5 and 5 are opposites:
Both 5 and -5 are five units away from the 0.
Positive numbers sometimes have a positive (+) sign in front of them, but can be written without (5 or +5).
Negative numbers must have a negative (-) sign in front of them, -5.