This is for planting traps with your 1 and then having a enemy get caught in them. This can be done over multiple matches but you can easily get this done in a single Arena match.
You can put down a maximum of three traps at a time, a fourth trap will get rid of the first one you placed.
Simply place three traps around the middle of the Arena map and people will either knowingly or unknowingly walk into them from time to time.
Going from the enemies perspective it's a good idea to walk into a trap on purpose when nothing is really going on so one of their allies won't get stuck later during a team fight. As you simply need the trap to be activated and nothing more this works in your favor.
In case you don't unlock this in a single match in order to check progress outside of a match go to your profile and then the "Achievements" section, next go to "Combat Practice" witch is the one with the two swords.