To donate means to give something — money, goods, or time — to some cause, such as a charity. The word has a more altruistic meaning than does simply "giving"; it suggests that you don't expect anything in return for the contribution.
The verb donate has always meant "to give," all the way back to the Latin verb donāre, "to give as a gift." Trace it back even further, to the ancient Indo-European roots, and you still have that same feeling to the word — "a gift." But it's more than a gift — it's a gift with the intention of helping. As an African proverb says, "If everybody in town donates one thread, the poor man has a shirt."