Coin Master Free Link is a feature that gives you the option to steal other players’ coins. This is often a friend, but could also be a random player or bot. I have encountered many bots with around 50 million each.
In raiding, there are 4 holes with 3 shovels to dig 3 holes. One of them is empty or has a bonus chest. The other 3 are filled with coins and one of them has more value than the other 2.
Foxy pet can be used to steal more coins. It must be fed with a treat to be active. Foxy will dig the 4th hole resulting in additional coins.
People can only raid a small number of your coins at a time. This varies depending on their star and village level. This is so you won’t get bankrupt by one raid if you have a hundred of millions or billions of coins.
Most people play by made up rules about not raiding friends before asked of permission. You don’t have to do that, but you will most likely get unfriended for “big-raiding” most people.
There is one strategy for raiding that I know called Two finger trick.