Added definition in 'understanding how to use PDBs' section.

Author: subhtk

modules/nodes-pods-pod-disruption-about.adoc

@@ -30,6 +30,9 @@ parts:
 
 [NOTE]
 ====
+`Available` refers to the number of pods that has condition `Ready=True`.
+`Ready=True` refers to the pod that is able to serve requests and should be added to the load balancing pools of all matching services.
+
 A `maxUnavailable` of `0%` or `0` or a `minAvailable` of `100%` or equal to the number of replicas
 is permitted but can block nodes from being drained.
 ====