Medicare Part A covers Hospitalizations, and Part B covers most other medical care, but when your expense for the year reaches "point A" the coverage stops and your expenses escalate (you pay it all) until your expenses reach "point B," at which time a slightly different coverage applies to any further expenses.
The range of expenses between point A and point B is refered to as "the gap," sometimes also called the "MediGap."
If you receive Social Security payments, enrollment in Part A and Part B are automatic and mandatory unless you specifically opt out of it by showing that you have another qualified policy. Few people opt out of the A & B coverage.
Most people will want a "supplemental policy" from a private insurer to cover expenses "in the gap," and in some cases to cover amounts in excess of what Medicare pays for covered items. (Prescriptions have a large impact for most people.) The supplemental policy must be "Medicare approved" and there are lots of different plans.
You can change policies only during the annual "enrolment period" except in a very few special cases. Any "Supplemental Policy Provider" should provide you with clear instructions about the limits and conditions that might permit you to make a midyear change, but for most people those conditions will be unlikely to occur. The "exceptions" are fairly consistent; but there may be slight variations between policies, so you might want to look at what they offer as explanations.
If you have an option to participate in a "retiree insurance" plan with a prior employer, it will usually be to your benefit to use that policy for your supplemental coverage, because most such plans are "bundled" with large insurers who are able to impose limits on what medical providers can charge for the various procedures/medications you might use (in place of, or in addition to the Medicare limits). Smaller insurers have lesser ability to impose such limits, so that even a policy that pays a "higher percent" or has a "lower deductible" may still leave you to pay a little more. That's one of the factors that makes choosing a supplemental provider much more complicated.
Most insurers will automatically continue the prior year election unless you choose to make a change, and after you've fretted over it for a couple of years you'll just look for the "If you wish to continue, there's nothing you need to do" notation in the annual notice and file it away - if you got what you want when you signed up the first time.
If you have only Part A and Part B coverage, you really should look at adding a "Medigap" policy, especially if you have - or expect - relatively high prescription expenses; but once you've made the decision for one year it should be continued without action in subsequent years.
The basic Medicare "information" for supplemental policies is "CFR42 Part 410 SUPPL MEDICAL INSURANCE (SMI) BENEFITS" which you can download from the Medicare website as PDFs; but if you want to look at the whole thing it's currently 79 separate PDFS (one per "chapter") with a total of 3.59 MB. There are a couple of smaller FAQs and "Information for new subscribers" pamphlets, but I wouldn't know which one(s) to suggest as a simpler starting point.
You should have received a small booklet called "Welcome to Medicare" that actually is fairly informative, and also a booklet titled "Medicare and You." Most of the explanations you should need probably are in those two, but you'll need to read them carefully to extract the information in terms that mean something for your own case. If you don't have them, you may be able to download PDFs from the Medicare website.