Face Frame

A face frame is a frame of solid wood -- rails and stiles joined at the corners, typically with pocket hole joinery, dowels, or mortise and tenon -- attached to the front edges of a plywood or other sheet-good carcase. It hides the raw edges of the sheet material (which don't finish attractively on their own) behind solid wood, and gives cabinet doors and drawers solid material to hang hinges and slides from, rather than the thinner, less fastener-friendly edge of a plywood panel.

Frameless (European-style) cabinet construction skips the face frame entirely, with doors and drawers mounted directly to the plywood carcase edges instead -- a cleaner, more minimal look, at the cost of needing edge banding or veneer tape to finish the exposed sheet-good edges cleanly, and generally needing more precise carcase construction since there's no frame to help true things up.