A mouse trap was a technique used while engaging an enemy starship whereby if the first attacking ship fails at destroying the target, there are two backup ships to do the job.
The Klingons employed this technique in the Delta Triangle when the IKS Klothos engaged the USS Enterprise. After the Klothos disappeared from view, during an engagement with the Enterprise, the crew of the Enterprise quickly discovered the other two ships nearby.
Spock described the ploy as "tactically well planned," because had the Klingons been successful, and when the Federation would investigate the disappearance of the Enterprise, the ship would have been recorded as "just another mysterious starship disappearance" rather than a fatality of the Klingon Empire. (TAS: "The Time Trap")