Marcha abajo is the quieter marcha: the gear for the cuerpo, without bells. The timbalero plays cáscara and the bongosero usually plays on the bongó itself, so the vocals come through clearly.
Marcha abajo is used mainly in the cuerpo, the verse part of the song. When the montuno starts with its coros, the band usually switches to marcha arriba.
Marcha abajo is the barer counterpart of marcha arriba. The biggest difference is in the bells: in marcha arriba both the timbalero and the bongosero play a bell ride; in marcha abajo those bells are not (yet) there. The bongosero then usually plays on the bongó itself, and the groove stays lighter and more open.
Marcha abajo is also a marcha: a steady groove to dance to with your partner. It is a good moment for calm, closed figures; save your energy for when the bells come in.