All pods are assigned a Quality of Service (QoS). These are 3 levels of priority pods are assigned within a node.
The levels are as following:
1) Guaranteed: High Priority
2) Burstable: Medium Priority
3) BestEffort: Low Priority
It is a way of telling the kubelet what your priorities are on a certain node if resources need to be reclaimed. This great GIF below by Anvesh Muppeda explains it.