It refers to meditation or a sense of radical self awareness. It is a stage prior to Samadhi. Dhayana is part of life. In fact, complete concentration of mind is called dhayana.
In Samadhi the Atma gets united with the paramatma. Samadhi state is attained when self awareness dissapeares completely too, feel divine pleasure after attaining the state of Samadhi. One experiences the virtual truth.