what is Thread Priority and its type

Thread runs on base of priority and threads will execute accordingly.Suppose If you might have created 3 threads say a1,a2,a3 in which a2 is most important for us and we give high priority to a2 and rest for low.Below are the some types of priority:-

(1)Highest

(2)AboveNormal

(3)Normal

(4)BelowNormal

(5)Lowest