Backup of Always On technology

Hi,

I have configured SQL server Always On in my sql server.

And the backup prefernence is  : Prefer Secondary.

The question is ,

I am not able to take backup on Seconday node in the availablity group.

I want to know the backup scenarios in always on technology.

As i am new to the technology, please explain about the backup preference.

Regards

Vinodh Selvaraj