Have you ever heard of people donating money to help others? Well, in Italy, instead of donating money, people can choose to donate a little bit of their taxes (money they have to give to the government). This is called the "otto per mille," which means "eight per thousand" in Italian.
Basically, the government gives people the option to choose where eight per thousand (0.8%) of their taxes goes. For example, if someone's yearly taxes are 500 euros, they can decide to give eight per thousand, which would be 4 euros, to a non-profit organization, or a religious institution, or a scientific research fund, or a political party or candidate, or simply to the state.
Donating through the otto per mille does not cost the person anything extra, because it's just a small part of their taxes that would otherwise go to the government. It's like if you had ten candies and you needed to give some to your friend. You could give them one candy and keep nine for yourself, but instead, you choose to give them 0.8 candies (which is eight per thousand of the total amount).
Some people like the otto per mille because they think it can make a big difference when lots of people donate little by little. Others don't like it because they think it's confusing and not everyone knows how to choose wisely where their money goes. But overall, it's just another way of helping others and using one's resources for a good cause.