The cruciate ligaments are the central stabilisers of the knee.
In this illustration these bones have been cut open down the middle so that you can see the full extent and direction of each cruciate ligament.
It is easy to see that the anterior cruciate ligament (ACL) holds the tibia from sliding forwards. The posterior cruciate ligament (PCL) holds the tibia from sliding backwards. The combined action of the two strong ligaments is to prevent 'antero-posterior' ('forwards-backwards') instability.
The cruciate ligaments stabilise the knee in conjuction with the other stabilisers like the collateral ligaments and the menisci.
The ACL versus the PCL
The ACL contributes more to the stability of the knee than the PCL. For this reason many PCL injuries are missed at first visit to a clinician, and even if they are not reconstructed they contribute much vaguer symptoms than do neglected ACL tears.