In physics, there are a few distinct uses for the word "relative".
Essentially, relative is used to contrast with absolute. If something is relative, you need to pick a reference point to measure it. Two people measuring the same thing at the same time will get different values unless they are using the same reference point.
The most common example has been known since Galileo. We call it Galileian relativity. The idea is speed is relative. When you're standing still on a sidewalk, a car zooms past you at 50 miles per hour. However, if you are traveling in that car, the car appears to be still and a man on the sidewalk is the one flying at 50 miles per hour.
Similarly, if you are traveling in a car, and you accidently rear-end the car in front of you, the amount of damage isn't determined by your speed relative to the road (~50 mph). Instead, it's determined by your speed relative to the other car (~2 or 3mph).
Einstein extended Galileian relativity in the early 20th Century, accounting for a strange property of light (that it always moves at the same speed regardless of your reference point). Some other consequences of Einstein's theory of special relativity are that not only velocity, but also distances, time spans, and simultaneous-ness are "relative" properties.