Open enrollment is a window during which individuals and employees may add or drop their health insurance, or make changes to their coverage. Under the ACA, individual health insurance will have open enrollment windows, which apply both in and out of the exchanges (in the past, people could apply at any time, but had to go through medical underwriting).
The initial window will be long in order to accommodate the expected influx of enrollees, from October 1, 2013 to March 31, 2014. In future years, the window will be shorter, going from October 15th to December 7th. Open enrollment can also be triggered by a qualifying event.