Relativity is a hard concept to define without concrete examples.
The simplest way is to say that from any point of observation, the view of the world will be different.
This applies to humans or something as simple as light measurement devices in different parts of a room. While the room may be lit by the same window or lamp, what one perceives depends on where one is (and one's perceptual abilities).
A political philosophy, relativism, is loosely related to this, but is a misinterpretation of it. Relativism attempts to impose an individual's subjective worldview as absolute while simultaneously affirming the same of all others. This is a conversion of an epistemological argument (we know the world through its representation) into a moral one (therefore we should judge all of them as equally true).
Relativity is important because it describes for us how the universe works in a sense outside of the absolute. There is no pure, single truth, but many different views of the same truth. And each one is changed somewhat by its position and the skills of the determiner.
With relativity, it becomes easier to see the invisible world of structure.