A cruciate ligament is a large ligament right in the middle of the knee. There are two of them - anterior (ACL) and posterior (PCL).

The word 'cruciate' means 'crossed', referring to the fact that the two cruciate ligaments cross over one another in the centre of the joint.

Here the bones are distracted so that you can see the attachments of both anterior and posterior cruciate ligament.
What do the cruciate ligaments do?
These ligaments are very important structures, connecting the thighbone (femur) and the shinbone (tibia) and stabilising the joint from frontwards and backwards forces.
Biomechanics of the anterior cruciate ligament: Physiology, rupture and reconstruction techniques Domnick C, Raschke MJ and Herbort M. World J Orthop. 2016 Feb 18; 7(2): 82–93.
Management of Anterior Cruciate Ligament Injury: What's In and What's Out? Raines BT, Naclerio E and Sherman SL. Indian J Orthop. 2017 Sep-Oct; 51(5): 563–575.