Some ICO projects doing airdrop campaign before launch their ICO. That mens they are giving their tokens for free. We have do simple tasks for participate the airdrop like follow twitter, Retweet, Join telegram group etc. Most ICO projects are ethereum based projects. You need ERC20 tokens supported wallet (Myetherwallet, Metamask) to get airdrop tokens. Some projects are based with NEO, Waves etc and some airdrop coins receive their own wallets too. You have to register their sites to get tokens.