Glossary
Bait
In one line: Sending a teammate (or yourself) in first to draw enemy fire and punish whoever shoots.
What it means
Baiting means exposing yourself first so the enemy shoots you and gives away their position: a teammate behind you sees them and kills them with the trade. Done well it is a normal tactic: whoever goes in first knows they may die, but the team gains the information and the kill.
There is also the negative meaning: to "bait" a teammate is to let them die alone on purpose, hanging back to grab a clean kill or to avoid risk. Then the team loses a player and gains nothing.
An example
Your duelist goes first into A main and gets killed by a defender in the corner. You are two steps behind, see where the shot came from and kill them right away. The bait worked: 4v4, and you know where they were.