What is Dana?

Essentially Dana means unconditional giving. Here is more from Wikipedia:

Dāna (Sanskrit: दान dāna) is a Sanskrit and Pali term meaning "generosity" or "giving". In Buddhism, it also refers to the practice of cultivating generosity. Ultimately, the practice culminates in one of the Perfections (paramitas): the Perfection of Giving (dana-paramita). This can be characterized by unattached and unconditional generosity, giving and letting go.