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The understanding of it is based in the concept of velocity. We use velocity against a fixed object (the Earth, usually), so we tend to think of it in very finite terms. An object's absolute velocity (say, a train leaving location x, to use the cliche) may be 120 km/h. However, if another object is heading towards it at 100 km/h, it's velocity relative to that object is actually 220 km/h. Once you understand that, it's a simple time/distance/velocity calculation.
You can do funny things with that knowledge. For example, if you're riding the train that's going 120 km/h and you throw a ball off the back at 30 km/h, the ball will appear to you to be travelling away from you at 30 km/h. However, to a stationary observer, the ball is actually moving in the same direction as you at 90 km/h.
Velocities are fun stuff.
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I wake up in the morning more tired than before I slept
I get through cryin' and I'm sadder than before I wept
I get through thinkin' now, and the thoughts have left my head
I get through speakin' and I can't remember, not a word that I said
- Ben Harper, Show Me A Little Shame
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