It means you have exceed the *Fair Usage Policy limit and the speed will be reduced for the remaining days in the current billing cycle.
*FUP limit restricts how much you can download in a billing cycle (yes this applies even if they call it 'unlimited' )
If you dont know what is your FUP download limit is ask Airtel ... or if your plan was doubled for free check if they sent you a email giving the details.
Thank you for the reply, genie. I just checked my email. the only emails I have from airtel are 'free night calling offers', 'loyalty bonus for airtel DTH', Landline bill etc., No email regarding my speed being doubled. But my speeds did get doubled. My last Bill Period 26/07/2010 to 25/08/2010, so I got double speeds from 26/7 to 25/8. I was careful about not downloading large sized files, but even then my free quota is over!! I'm on 1299 turbo plan. I was getting 1 mbps in the day and 2 in the night. so it's 512 in the day and 1 in the night now. when will my speed double again then?