Anything firsthand is directly experienced, so your firsthand knowledge of the new math teacher's purple hair comes from the fact that you've seen it with your very own eyes.
When you get information from someone who saw something happen or heard something said, that's firsthand evidence. And anything you see or hear yourself is also firsthand. Your summer job might be a firsthand look at what farming is really like. Interviewing a witness to a crime gives a police officer firsthand testimony about what exactly happened. This word, coined around 1690, comes from the idea that the maker of something is its first hand.