The physical constants are measurable properties of the physical universe that don’t change, everything else is relative to these constants.
Relativity refers to Einstein’s theories of general and special relativity, Galileo’s Galilean relativity, Sir Issac Newton’s Newtonian relativity, and the work of all the other physicists who helped complete these theories (like Maxwell and Lorentz).
Galileo and Newton (and many others) showed how physics applied to earth (an inertial frame), and Einstein (and many others) showed how it applied to the universe and at high speeds (an accelerated frame).
In a overly-simple nutshell, Galileo found that motion was relative, Newton found that force was relative, and Einstein showed how time and space were relative to speed and gravity (relative to the constant the nature of mass-energy, in which mass and acceleration result in “spacetime” curvature, which warps space and time, and thus needs to be accounted for when we calibrate instruments).
All types of relativity depend on frame of reference, or “point of view”. As from any point of view an observer will always measure the physical constants, including the speed of light, Planck’s constant, and Newtonian gravity, the same. This is why the phrase “relative to the observer’s reference frame” is used and also why the physical constants are important to physics.
See PBS’s SpaceTime series for a great video series on relativity (highly recommended and liberally featured on FactMyth.com), or simply check out our list of facts and myths on relativity below.