Activation lock was added by Apple to prevent thieves from using of stolen iPhones/iPads/iPods. It requires that the person whose account it is linked to remove the device from their account before anyone else can activate the device. You will have to contact the previous user and have them do it or give you their login info so you can do it yourself. It should only take a few minutes to remove the iPad from the old account.
Guide from MacWorld:
"[Y]ou'll need to get their password from them (or get them to enter it). If you don't have their password (because you bought the phone second-hand) then you should get in touch with the person as soon as possible, or return the iPhone for a refund.
Once you have the password, follow these steps to remove the Apple ID account from iCloud and Find My Friends:
1. Tap Settings > iCloud.
2. Scroll down and tap Sign Out.
3. Tap Delete from My iPhone.
4. Enter the password of the person's Apple ID.
5. Tap Turn Off.