Design Principles are a tool for creating a better, more consistent experience for your users. They are high level principles that guide the detailed design decisions you make as you're working on a project.
There are essentially two kinds of them. Universal and Specific.
The universal kind, also called Heuristic applies to basic human behaviors and can be used for all kinds of designs. Examples of Universal Principles are: Jakob Nielsens famous 10 Usability Heuristics for User Interface Design and Whitney Hess's 20 Guiding Principles for Experience Design.
The other kind of Design Principles are the Specific ones You create these for a specific project to ensure a consistent design. Examples of Specific Design Principles are Tivo's Design Principles and the Android Design Principles. The Specific Design Principles are especially great when you're working on a big project with a lot of people involved. By establishing a set of Design Principles it's easier to keep the product aligned with the UX Vision.
If you're not doing it already, you should start using Design Principles to improve the User Experience in your own projects. To get started check out the Design Principles of others to get inspiration for your own.