Can anyone help me regarding association bonus?
What is this association bonus anyway?
StackExchange is a network of multiple Q & A sites. When you register for more than one site, you can do so using the same account (I see that you have done that).
When any of your linked accounts reaches 200 reputation points, all of the linked (or "associated") accounts receive a bonus of 100 reputation points - going from 1 to 100 grants a handful of privileges, including commenting and flagging.
It is kind of a "thank you, we now know that you know how to use the network, so we're granting you these privileges on all sites where you participate."