For the uninformed a "signing statement" is a writ the President can issue when signing a law saying, "I will sign this bill into being the law of the land, but I will not enforce it for the remainder of my term of office."
Not really.
The administration enforces the law, but in order to do so they must interpret the law. In certain cases the president will choose to issue a statement when he signs a bill explaining what his interpretation will be, especially when a law is vague.
These are similar to legal memos and other documents that, within the government, define what is legal for the administration to do. Unless the courts get involved the administrations interpretation of the law
is the law, simply because that is what dictates how the organs of government actually enforce/carry it out.
A fairly common use is when the administration is lumbered with some provision they hold to be unconstitutional or otherwise unsuitable to enforce. They can effectively ignore such provisions. A fairly recent example was certain provisions within the
april defence budget, containing similar restrictions on funds related to GTMO as this new bill. Obama's statement
outlined his constitutional case against the provisions while still allowing the bill to pass into law.
Arguably signing statements aren't required at all and are only the most public example of administration authority to interpret and enforce the law. What they tend to be used for is establishing a (weak) form of legislative history. The administration can choose to treat laws as unconstitutional without needing a signing statement. A fairly recent example would be Obama choosing not to defend DOMA where there is a legitimate constitutional challenge to be made.
Anyway. The issuing of a signing statement itself isn't a big deal, especially on bills where the language is vague and there is a high measure of administration interpretation involved. Hell, it may be a purely rhetorical or political statement with no explicit objection to any of the provisions.
My first guess is this is targeted at the GTMO funding language, same as that other bill linked above. The argument is almost identical to that one. As for anything on the detention provisions, probably something about the mandate not actually being a mandate (which it sorta isn't anymore anyway). Anything else will be interesting.