The best and most reliable sources are primary sources, as it is an eye-witness account of what occured. Things like diaries, autobiographies, recordings, artifacts, or an interview with a witness are examples of primary sources.
Secondary sources are things, such as a reporter telling someone what the witness told them, as the reporter could have changed the story a little without telling anyone after they interviewed someone.
For example, a reporter may ask a person who saw a car crash, and the reporter wants money. So the reporter changes up the story and tells the publishers, who print out the newspaper with the reporter's story, or the publishers changed up the reporter's story and put it on the newspaper.
The newspaper may now say: BREAKING NEWS! A DRUNK BUZZARD GOT INTO EPIC BATTLE WITH VIDEO GAME CHARACTERS!!!!
-which of course didn't happen-
It is important to get credible and reliable sources for research, because you can trust what happened, and you can use that information for your work without having a seriously biased result or conclusion.