Hey there!
The distance on a number line is the distance that a point is from 0.
For example, if you were looking at a number line and there was a point on the number 5, that point would be 5 units away from 0.
Opposite numbers are related because they are the same distance away from 0. For example -3 and 3 are both 3 units away from the number 0.
I hope this helped! :-)