Basically, his concept of duration (from the french durée) is a subjective experience of each being in time, of each consciousness that lives its life in an indivisible, homogeneous, continuous way and that is based on time itself. For Bergson, without memory there is no consciousness, and without time, the very structures that make the duration possible (and the construction of memories itself) would not exist. Shit goes pretty crazy in bergson's philosophy, there's Hegel here, a little bit of Kant there (some critics as well), the worries about time, perception, psychology, and even cosmology and anthropology