From Unofficial BOINC Wiki
A "Triplet" is a set of three equally spaced Spikes of roughly equivalent power. The SETI@Home Science Application tests for Triplets by looking at every pair of Spikes above a certain threshold power. It then looks for another spike precisely between the two spikes. If one is found, a Triplet is logged and sent back to Berkeley.
The "power" tells us how strong the Triplet is, relative to noise. The "period" (the elapsed time between the spikes) tells us how far apart the three spikes are (in seconds).