What are the postulates of special relativity?

There are two postulates of special relativity:

1. The laws of physics are invariant in all inertial frames of reference.

What this means is that if we have a description for how physical systems undergo change in one frame F, then that should remain the same in another frame F' as long as F' is only moving at a constant velocity relative to F. Note that a frame of reference is just a set of coordinate axes against which we can measure positions in space and time. Inertial means that it is non-accelerating.
2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

This means that regardless of how the light source is moving with relative to the observer, the speed of light will be measured as a constant c.

From these postulates we can then derive the consequences such as length contraction, time dilation, universal speed limit etc.