What do you mean by sending only contacts every other airdrop?

Send only contacts every other airdrop refers to contacts in the address book. The "Contacts Only" option for iPhone airdrops is determined according to the contacts in your address book. If you select this option, only the contacts in your address book can see the files you send at the receiving end, and only they can send files to you. If this option is turned off, anyone can send you a file, but the recipient must accept the sender's request to receive the file.