What makes everything in Einstein's theory relative

There's two portions to relativity theory. One is special relativity, the other is general relativity.

General Relativity is related to warping of space time due to gravity and I'm not quite familiar with it.

More commonly understood is Special Relativity. This basically says that length is longest and time is shortest in the inertial reference frame of the object being measured. (Inertial reference frame simply means it is at a constant speed and not accelerating - not feeling any ficticious forces such as the sensation you feel when your car turns a corner) If you're standing on a train traveling very fast (only very near light speed these changes become obvious) and someone is on the platform, you will seem shorter in the direction of train travel (the same height but a lot thinner), and you will seem to age very slowly to the man on the platform. Length is longest and time shortest where you are, so when someone else looks at your fast moving self, length can only get shorter and time longer. The paradox happens when you consider yourself looking at the man. Because you are traveling at a constant speed, you may also think of yourself as stationary, and the rest of the world and the man traveling at that high speed away from you. The man now looks squished and ages slowly. Who is right?! The point is that it doesn't matter who's inertial reference frame you're in. The laws will be the same no matter where you are. Time is always shortest, and length longest in the object in question's frame. Unfortunately, you cannot use this to extend your life span as some simpler examples may indicate, as you yourself will always feel the passage of time the same. People may age faster than you so you may seem to live longer to them, but you feel time the same.