In order to take measurements in mechanics, an observer needs to choose an origin in the time scale, an origin in space and a set of coordinate axes. These three items form collectively what is referred to as a frame of reference. The location and time an event occurs can be specified by a set of coordinates (x,y,z) and a time t within this frame.
Some frames of references are chosen such that Newton's first law is obeyed within them. These unaccelerated frames are known as inertial frames of reference. The laws of physics are the same in all inertial frames of reference, e.g. Newton's laws apply to all inertial frames.
