This is just a formal Attorney General
Opinion. It in no way, shape or form means that Maryland now recognizes same-sex marriages from other states.
Writing and issuing AG Opinions is one of the functions of the Attorney General's office. A government official (e.g. the Governor, a state legislator) asks it for an opinion on some legal matter, and it researches (if necessary) and writes an opinion in response. In this case, a State Senator asked
'whether Maryland may recognize same-sex marriages legally performed in other jurisdictions' and
'whether a Maryland Governor can issue an executive order concerning recognition of such marriages'.
The Attorney General issued a lengthy opinion discussing various aspects of the issue. The short answer to the first question was yes - same-sex marriages
may be recognized under state law.
Speaking to how the Court of Appeals might rule on the issue:
And, speaking to the second question:
A few key points:
(1) The formal opinion offered by the Attorney General was in response to an inquiry (and offering such opinions is part of the AG's job).
(2) The opinion doesn't change state law or control Maryland policy on this issue.
(3) This isn't a suggestion as to what the Attorney General thinks Maryland policy should be, it is a legal analysis of the issue, and considered views regarding it.
(4) There is nothing 'effective immediately' to this - unless you mean, 'effective immediately' you will be able to read this opinion.