The term "reliable sources" gets thrown around a lot. While Wikipedia has a definition of a reliable source, many editors have their own.
An object everyone should strive for is to be one.
How to be a reliable source
The best way you can be a reliable source is to strictly adhere to the guidelines pertaining to them. This means to cite all information you add to articles, to be sure all information is verifiable, and not to include original research in your additions. If you get known for being a reliable source--that is, for using authoritative sources published by reputable publishing houses, it is more likely that your edits will be trusted.