Immanuel Kant believed that good will comes from inside and is not forced on a human being by anything external. He also considered the "good" to be fairly universal. By "without limitation," Kant means that, out of all good things in the world, only good will exemplifies "goodness" as an absolute. It's the only thing that cannot fail in being good.
A lot of things can be described as good, but all of them can falter, so to speak. For example, Kant is very careful not to say "good act" instead of "good will." If a person truly wants to do and be good, without any need for acknowledgement or rewards—that is Kant's good will. It's selfless, motivated only by the inherent moral drive.
Everything else that follows is already subject to failure and dispute. Take, for example, a person who wants to do good. Other than their desire to act morally, everything could go wrong. They could choose the wrong way to go about it, they could help out by accidentally causing someone else harm, or they could even do a good thing and then be struck by unforeseeable bad consequences. Acts have limitations and so do people. There are so many variables in the real world that almost nothing is untouched by subjectivity; what's good for one may not be so for another.
In Kant's mind, in that whole equation, only the good will remains pure and untouchable. Even if it's misguided, or if it fails in execution, the drive to be and do good is the one thing that can't be corrupted—that can never be bad.