A smear (or "smearing") is a foot technique where the climber presses the sticky rubber of their climbing shoe flat against the wall — using friction alone to hold the foot in place, with no positive foothold. It is the workhorse footwork on slab climbing, where features are too small to provide proper edges and the climber relies on rubber compression and body position.
Good smearing has two requirements. First, weight has to go through the foot — push the foot into the wall hard, lean back away from it, and trust the rubber. Second, the heel should drop low and the toes flex up so that the maximum surface area of the shoe sole contacts the rock. Lifting the heel reduces contact and is the most common smearing mistake.
Fresh, clean rubber smears far better than old or muddy rubber. Aggressive bouldering shoes with stiff soles smear poorly because they don't flex onto the wall — flatter, softer shoes are far better for slab and smearing-heavy routes. Smearing is one of the most important skills to develop early, because it teaches trust in the feet.