When two V 32 bis modems talk to each other at 14,400 BPS they make 128 different possible sounds to each other The sounds are various combinations of phase shifts, and amplitude (loudness) levels of the basic carrier tone The modem changes the sound 2400 times a second (hence 2400 baud) You could imagine a diagram of the possible sounds arrayed like a lattice or trellis of intersections of phase and amplitude
A trellis is a frame which supports climbing plants. a frame made of long narrow pieces of wood that cross each other, used to support climbing plants (treliz , from trilicius , from tri- + licium )