The word moral in our language means right action. Hume does use the same meaning (1), hence his work An Enquiry concerning the Principles of Morals. But he also uses the word moral in two more general senses: (2) Pertaining to human action in general, and (3) Pertaining to the ground and explanation of human action, that is to “human nature”, by which Hume refers to what we would now call human psychology (innate tendencies, habits, mental dynamics).
At the beginning of An Enquiry concerning Human Understanding Hume writes:
Moral philosophy, or the science of human nature, may be treated after two different manners ...
The one considers man chiefly as born for action … pursuing one object, and avoiding another …
That is sense (2) of moral. Then he moves to sense (1).
As virtue, of all objects, is allowed to be the most valuable, this species of philosophers paint her in the most amiable colours …
Then to sense (3)
The other species of philosophers consider man in the light of a reasonable rather than an active being, and endeavour to form his understanding more than cultivate his manners. They regard human nature as a subject of speculation; and with a narrow scrutiny examine it, in order to find those principles, which regulate our understanding, excite our sentiments, and make us approve or blame any particular object, action, or behaviour.
(1-2)
As to what Hume writes on the two types of reasoning,
All reasonings may be divided into two kinds, namely, demonstrative reasoning, or that concerning relations of ideas, and moral reasoning, or that concerning matter of fact and existence.
(30)
He uses the word moral in sense (2), pertaining to action in general, as he explains later.
Such is the foundation of moral reasoning, which forms the greater part of human knowledge, and is the source of all human action and behaviour.
(132)
In other words, moral reasoning means the kind of reasoning that guides all human actions, at least whenever humans act prudently (hence “rationally”). Even though, as Hume famously argues, unlike “demonstrative” reasoning, “moral” reasoning is to a large extent irrational, grounded in psychology rather than in reason, essentially involving our innate tendencies and habits.