The 'verge and foliot' is the earliest real form of mechanical escapement. The crown wheel (c) is powered by a falling weight and moves a little way until it is in contact with an arm (pallet p) of the verge (v). The verge then spins, which momentarily releases the wheel until it comes into contact with the second pallet (q). This action is controlled by the 'foliot', a crossbar at the top end of the verge. The weights at each end of the foliot can be changed to alter the timing.
The pictures at Wikipedia
help to explain it.
The verge and foliot escapement was difficult to regulate, one cause being the expansion of the foliot mechanism with an increase in temperature.
The foliot escapement was later replaced by the balance wheel.