Firsthand means learning or experiencing something directly yourself, rather than hearing about it from someone else. When you have firsthand experience with something, you were actually there when it happened.

If you visit the Grand Canyon and see its massive cliffs and winding river, that's firsthand knowledge. Reading about the canyon in a book or hearing your friend describe it would be secondhand information: real and possibly accurate, but filtered through someone else's eyes and words.

Firsthand experiences are especially valuable because they let you form your own understanding. A scientist conducting an experiment gains firsthand data. A reporter interviewing witnesses at an event gets firsthand accounts. When you tell someone “I saw it firsthand,” you're saying you witnessed it yourself, not that you heard it from your cousin who heard it from a friend.

The opposite of firsthand is secondhand, which means information that has passed through at least one other person before reaching you. That's why eyewitness testimony (what people saw firsthand) matters so much in court, and why teachers value firsthand research over just copying what others have written.