Here are some explanations of the power in 'pounds or lbs':

Once the trigger is pulled, the bolt of the crossbow (also called 'quarrel') is propelled by the string. Īnother advantage with a crossbow: the bolts are not made according to the shooter's morphology. With a crossbow you always have the same power in the tension of the string and you can even shoot lying down. In its principle it is a bow placed horizontally on a tiller, (the body of the crossbow) and whose mechanisms (a built-in mechanism that holds the drawn bow string, as well as a trigger mechanism) that allow the constant tension of the string, so that there is no longer any effort required to hold the string as on a bow the shooter can concentrate on his shot and the accuracy is easily improved. What is the principle of the crossbow? How is it used? What are its characteristics? What is the speed of a crossbow bolt? This is what we will see here. Today, among the modern crossbows we find different models with very innovative technologies. Over time, their mechanism and design have evolved tremendously.

The first crossbows appeared in China in the 5th century BC.

Mount and blade wiki crossbows