The overhand knot is the most basic knot in any rope work tradition. You probably learned it as a child without realising it had a name. It is formed by making a single loop in the rope and passing the working end through that loop. The result is a small bulky knot that prevents the rope from passing through a hole or running through a device.
In climbing, the overhand knot has three main uses. First, as a stopper knot at the end of a rope (preventing the rope from passing through a belay device). Second, as a backup behind a primary knot like a bowline (preventing the bowline from shaking loose). Third, as the building block for more complex knots — the figure-8, the fisherman's, the double overhand, and many others all start with an overhand and add complexity.
For climbing tie-ins, modern instruction prefers the figure-8 follow-through over the bowline-with-overhand-backup. The overhand-on-its-own can also be used as a backup tie-in but is generally less reliable than the figure-8 — overhands can untie themselves under cyclic loading more easily.