This is a really simple way to understand:
1. You take people to the shop
2. People buy something in this shop.
3. Shop gives you reward ( about 1-10% of money that your people have spent ).
This example works fine in real world.
BUT!
In internet this works another way. You can take other visitor to the shop, visitor buys some products but shop can tell you that this wasn't your visitor. Case is that after your visitor entered the shop he will be surfing the internet and other people can take this visitor to the shop. And the last who will send this user to the shop will get the reward.