The simple answer, it’s literally just free coins just waiting for you.
The method behind this is a decision regarding marketing strategy. As a way to spread awareness to the relevant audience of potential investors and eventual enthusiasts, coin teams will, from time-to-time, do airdrops. As a method to pump coin value, coins would announce airdrops where coin holders would receive bonus coins proportional to the amount of total coins they hold. Coin investors that wanted to get the airdrop would have to buy the coins if they didn’t have any yet, or buy more if they wanted a larger proportion of the airdrop. Participating in an Airdrop is simple. You discover, or are informed of an Airdrop, fill out a telegram form, give your Ethereum (or relevant coin) wallet address, and Voila ! Free Tokens in your wallet a few weeks from then. You can find airdrops around the internet, just search for them.
What is it also does is create marketing waves in the crypto ecosystem. People start discovering and talking about “free” tokens, and the word spreads around the community about that particular Token. The word reaches thousands of people, and the cost of advertising – zero.