The 5th limb in Patanjalis 8 limbs of yoga. And that is Pratyahara. From a really basic understanding, this means sense withdrawal. 
Most of us spend the majority of our life in the external world, meaning that we are concerned with things going on around us. We don’t spend as much time in our internal world, focussing on the workings of our bodies and minds. However to gain any sort of traction in meditation it is important to to be able to withdraw our minds from the external environment.