Something that's excusable can be justified or easily explained. Being late to school because of a dentist appointment is completely excusable.
If there's a good reason for a mistake, it's excusable. When you're able to forgive someone for an offense or error, it is also excusable. A little boy's extreme excitement on his birthday is excusable, and lying around all day watching TV is excusable when you're home sick. The adjective excusable has a Latin root, excusare, "excuse, apologize, or release from a charge."