From Minecraft Wiki
Modifies a scoreboard objective with a "trigger" criterion.
trigger <objective> <add|set> <value>
- Used together with
/tellrawto let players activate systems made by operators or mapmakers.
- An enabled scoreboard objective with the "trigger" criterion.
add– Adds value to the current value of objective.
set– Sets the value of objective to value.
- Fails if the arguments are not specified correctly, if objective does not have the "trigger" criteria, or if the player who executed the command is not able to modify objective.
- On success, the value of objective is changed for the player that executed the command.