Beitragsbemessungsgrenze der gesetzlichen Krankenversicherung - is the brutto yearly salary limit for the compulsory medical insurance. If your salary is below this limit, you MUST be medically insured in "gesetzlich" medical insurance (like AOK, Techniker Krankenkase, ...). Approx. 90% of the people in Germany are insured in that way. If your salary is above this limit you can a) do not pay for the medical insurance at all (not much people in Germany do that, but I know some) b) insure in some "Private medical insurance" like "Barmenia" for example (but there are many others), you pay generally less than in "AOK" or "TK" if you are young and healthy and single. But if you are married and you have kid(s) and women, and women does not work you pay FULL amount for each one of them, which is expensive. c) you stay at "TK" or "AOK" but you are now insured as "freiwillig" (on your own wish) - but you do not pay more than some maximum limit, even if your salary is very high.
What has this to do with immigration ? This "Beitragsbemmesungsgrenze" is some kind of the limit between "average" and "high" salaries. It is at the moment PROBABLY around 45000 euro brutto (the number might be wrong, I event think that it is little bit more at the moment. So if you get the double of this limit, you must be having "high" salary and you are "specialist" and you get PR.