Dana is the Pali word for generosity. For weekend and longer retreats the teachings are offered freely and the participants are given the opportunity to offer a donation at the end of a retreat. It is an acknowledgement of our interconnectedness and interdependence.
The fee you pay is for food, organisation and accommodation and these costs are kept as low as possible so that anybody, regardless of financial background, can attend. With the donation (Dana) you express your appreciation for the teachings you have received and your support for the teachers to continue their work.
"The practice of giving is universally recognized as one of the most basic human virtues, a quality that testifies to the depth of one's humanity and one's capacity for self-transcendence. In the teaching of the Buddha, too, the practice of giving claims a place of special eminence, one which singles it out as being in a sense the foundation and seed of spiritual development."