This is a term that reminds most (well, most people who have had something to do with science) of Newton, physics and mechanics. Strictly speaking, inertia is the resistance an object has to a change in its state of motion. If a body is moving, it will keep doing so till an external force tries to stop it. Same way with a body at rest.
Now, how do you define rest? It really depends on the frame of reference right? If the object does not change its position with respect to this frame with time, it is 'at rest'. Now, if this frame of reference is moving (with respect to another boss-frame of reference of course), the 'stationary' object isn't well, stationary.
Similarly, an object that is perceived to be moving in one frame of reference can be taken as stationary in another. It all really depends on what the observer assumes to be the basic frame of reference. Where am I going with this explanation of high-school physics? Well, try applying this to 'real life'. Day-to-day situations. You are slogging your behind off at something, while your superior thinks you aren't moving at all. And things happen the other way round sometimes.
So if reality is just a bunch of perceptions, what is constant? Or in other fancy words, what remains unvarying between all the intersections of varied perceptions we label truth? And if it all really is this futile, why bother at all?
I am not so sure I know the answer to the first question. I'm guessing one word answers the second.