Two characters grind their hips together for friction while they stay clothed or at least keep underwear on, so it is all rubbing with no penetration. The action usually focuses on pressing a hard dick against fabric, groping, and the slow buildup until there is a damp spot, precum, or an orgasm through the clothes. It tends to read as teasing foreplay that can either stop at grinding or escalate into full-on fucking in the next panels.