Differences between value of Allocatable and Capacity in Node Status

Issue

• Why the Allocatable pod number and Capacity pod number are equal ?

# oc describe node
...
Capacity:
 cpu:     4
 memory:  4045508Ki
 pods:    40
Allocatable:
 cpu:     4
 memory:  3943108Ki
 pods:    40
...

• What does each value in the Node Status mean?