Glossary
First blood
In one line: The first kill of the round, which creates the first numbers advantage.
What it means
First blood is the first enemy killed in a round. It matters a lot: the team that gets it plays 5v4, with an extra player to hold, attack or trade. Teams that win the opening duel win most rounds.
That is why you do not give the first kill away: dying first for no reason (a dry peek, running into an unchecked corner) puts your team on the back foot. For the same reason, a first blood taken with a huge risk is not always worth it.
An example
At the start of the round a defender waits in an unusual spot and kills your first player running down main. Ten seconds in, you are 4v5: now you have to play more carefully, or change the plan.