In these games, such as Call of Duty (pre modern warfare 2), one of the players in the game is randomly and unknowingly given the host connection. If this player leaves the game, it ends the match for everyone :(
When a player is fed up and tired of getting owned, they may decide to leave the game. When doing so, one might exclaim they are going to "find out if they are the host", referring to the possible outcome when they leave the game and as a result the game ends for all the players.
The term refers to committing suicide, as in similar frustration when leaving a video game.
(real life application)
"Yo bro, I just got a 15 on that math test, I'm about to find out if I'm the Host"
"Wow some kid just walked over to me and said 'safety', i'm about to find out if i'm the Host"
"My best friend has been secretly sleeping with my girlfriend for 3 months now. Im about to find out if Im the Host"