A direct descent is a mission profile where a spacecraft lands on a planet without achieving orbit first. To perform one, intercept the target body on an orbit with periapsis below its surface. A few km before impact (depending on your thrust-to-weight ratio), burn retrograde to kill all speed. The right burn altitude can be calculated using the specific orbital energy and vehicle's acceleration. Better add a
fudge safety factor - a little extra altitude and fuel - because nobody's a perfect pilot (except maybe MechJeb). Using the rocket equation will give a better answer, if a large amount of propellant relative to starting mass is needed. Kerbal Engineer has a readout to help with this.
Nicknamed a suicide burn for how easy it is to miscalculate and crash. Direct descent landings save some fuel over landing from low orbit, but usually not enough to be worth the added difficulty and risk.