Effective edge is the length of your board's edge that actually makes contact with the snow when you're carving a turn — it doesn't include the curved-up tip and tail areas. Two boards with the same total length can have very different effective edges depending on their shape and camber profile.
A longer effective edge gives you more grip and stability at speed, which is great for carving hardpack or charging big mountain lines. A shorter effective edge makes turns easier to initiate and spins quicker, which freestyle and park riders often prefer. Keep in mind that rocker boards typically have a shorter effective edge than camber boards of the same length, since more of the tip and tail lift off the snow.
When choosing, think about your priorities: if edge hold and high-speed confidence matter most, lean toward a longer effective edge. If you want something nimble and spin-friendly, a shorter effective edge is the way to go.