Something that's charitable has to do with helping people who need assistance. A charitable act might include volunteering at a soup kitchen or homeless shelter.
A charitable donation is money you give to an organization that helps needy people — and the organization itself is also charitable. You can also use the word to describe, more generally, someone who's kind and generous, or the quality of giving people the benefit of the doubt: "He always had such a charitable opinions of me." Charitable comes from charity, from the Old French charité, "mercy or compassion."