Special Enrollment is the health insurance period that lasts for most of the year (February 1- October 31 for 2018) and requires applicants to have a qualifying event in order to apply for health insurance.
Special Enrollment was created through the Affordable Care Act, primarily as a compromise in issuing the mandate that health insurance companies can no longer deny individuals coverage for preexisting conditions. In other words, individuals are guaranteed coverage regardless of their health status.
If individuals could simply start and stop coverage whenever they wanted, many people would simply wait until they got sick or required expensive medical care before buying insurance. That is why the government has limited the time to purchase health insurance without any qualifying event to 3 months out of the year.
If you don’t have a “qualifying event,” you will not be able to purchase until the next Open Enrollment, which will be from November 1, 2018 - December 15, 2018.