There are 3 kinds of airdrops
1. Devs expanding supply - this has happened with a few cryptos and basically if you hold a certain amount of crypto an additional amount will be added just because. You need to take no action to receive this.
2. Devs expanding the network you're already on - another coin has been issued on an existing network (historically mostly ETH based, a few others including XRP have happened). It will be a different coin/crypto, you'll have to put your wallet address and follow up with procedures to set up a separate crypto account after. Usually fine but be wary with your security - if they ask for passwords of any sort it's 100% scam. Do some research, many are genuinely interesting projects but don't compromise your holdings by revealing too much info. If it seems fishy it probably is.
3. Most common nowadays since the ERC20 rage has died out are plain old scams. You'll see swarms of posts all over twitter and other social media of bots claiming to receive large amounts of crypto. It's click bait trying to steal your info and is easy to spot. Again just do some research before clicking on weird links, a quick google search will clear everything up. If someone claims to have received over $100 worth of crypto as they usually do it's 100% ********.