The quotation is generally speaking accurate as an interpretation of Kant's account. At 4:393, Kant explicitly says nothing is good except a good will. There's a problem in Kant interpretation (I can find the papers if need be) that centers around the fact Kant never gives a clear definition of "maxim," but along a major interpretive line you add together the following details:
(1) Kant defines a maxim as a "subjective principle of volition" (4:401). The problem is that this defines basically nothing.
(2) Nearby Kant speaks of the maxim of the will. And describes this as the reason one acts. Note that we may be tempted to uses the word motive instead but this would be mistaken with Kant's moral psychology. Here, Kant considers what are on most interpretations four possibilities (on some only three) at 4:397:
a) one chooses to act in a way that cannot be universalized. this lacks moral worth as it fails the test.
b) one chooses to act in a way that mirrors what is good but a reason that is bad. This lacks moral worth.
c) one chooses to act in the right way for the reason of being a good businessman (i.e., not cheating one's customers to get them to return) but with awareness that one's action can be universalized. On the list of 4 interpretation, this lacks moral worth.
d) one chooses to act in the right way but is emotionally cold / contrary to the action he chooses. This has moral worth.
[The question is whether (c) and (d) are to be understood as different or merely two descriptions of the same person.]
Immediately after that Kant explains that actions are moral based on their purpose (4:399) which is their principle of volition.
(3) In the formula of universal law articulation of the Categorical Imperative, Kant states act such that maxim of your action could become a universal law for all rational beings.
The interpretive problem is how to (1), (2), and (3) to play nicely. The usual answer is not far from the wikipedia entry you quote though it elides the mechanics. A maxim is understood as I [as a rational being] chose to do X for reason Y. This maxim is the universalized to all rational beings choose to do X for reason Y, and this is tested for whether it is internally incoherent or compatible with a world. The missing piece is how to universalize maxims (i.e. what stays and goes from a maxim like "I will play tennis on Sunday mornings while my neighbors are at church and the courts are full OR I will tell a lie to a woman named Herbert on February 2nd at 3pm" (both examples from Allen Wood -- though I may have the details wrong on the second one)
- Kant, Immanuel. Groundwork of the Metaphysics of Morals.
- Korsgaard, Christine. Creating the Kingdom of Ends (1996)
- Wood, Allen. Kant's Ethical Theory (Cambridge University Press 1999)