Dana (pronounced "dah-na") is a Pali word meaning generosity. Dating back to the time of the Buddha, there has existed an interdependence between those who offer the teachings and those who receive them. The teachings are given freely, since they are considered priceless. Typically, the registration fees for retreats cover food, facilities, transportation, and other expenses. None of this money goes directly to the teachers (and sometimes not to the managers or cooks either). To allow the teachers and others to continue their dharma work, support from the students is needed. Most often, there is an opportunity to contribute at the end of each retreat.
According to the Buddha, generosity, or sharing what we have, is one of the central pillars of a spiritual life. In the act of giving we develop our ability to let go, cultivate a spirit of caring, and acknowledge the interconnectedness that we all share. The Buddha created a system to develop this quality of open-handedness, whereby those who share the teachings are dependent on those who receive them.
This explanation of dana was adapted from Spirit Rock.