This one is on something called Frames of Reference.
In Physics especially were motion is concerned, which is just about everywhere, you will often hear the term "Inertial Frame of Reference" or "Inertial Reference Frame", but what is it? Well, technically it can be described as follows (this one is taken from Wikipedia);
In physics, an inertial frame of reference (also inertial reference frame or inertial frame or Galilean reference frame) is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.
All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not accelerating in the sense that an accelerometer at rest in one would detect zero acceleration.
It goes on to say
Measurements in one inertial frame can be converted to measurements in another by a simple transformation (the Galilean transformation in Newtonian physics and the Lorentz transformation in special relativity). In general relativity, in any region small enough for the curvature of spacetime to be negligible one can find a set of inertial frames that approximately describe that region.
Glad we cleared that one up!