MAGI is Modified Adjusted Gross Income. MAGI is important because it is the income that is used to determine subsidy eligibility. I have a one page document courtesy of UC Berkeley Labor Center that is a great starting point and is sufficient for most people.
For those of you that have more complex situation or are simply curious. the best resource I have found is The Advocate’s Guide to MAGI by the National Health Law Program. It is almost 100 pages of in depth information on determining MAGI for complex situations.