A Special Enrollment Period lets you make a change outside the annual window when certain things happen in your life. The rules are specific, and the window is usually limited, so the important thing is to act when the event happens rather than months later.
This is the one most people are never told about. Having both cards can let you review and change your plan more than once a year, rather than waiting for the fall. More on that here.
Windows are time-limited and the clock usually starts at the event, not when you find out. If something on this list has happened to you recently, it is worth a ten minute call to find out whether a window is open and how long it stays open.
General information only goes so far. What is actually offered where you live, and whether it fits your doctors and prescriptions, takes about ten minutes to check. No charge for the review.