It is first mentioned in Section II of Groundworks of the Metaphysics of Morals. Kant defined it as the formula of the command of reason that represents an objective principle "in so far as it is necessitating for a will", in other words, imperatives act as the empirical formulas for knowing and enacting with reason. Hypothetical imperatives tell us how to act in order to achieve a specific goal and the commandment of reason applies only conditionally, e.g. "I must study to get a degree." These sort of actions are capable of producing good, but they are primarily motivated by a desire to meet specific purposes. Actions done via Hypothetical Imperatives are done very often; whenever one commits to perform an action in order to achieve something they desire, they have utilized this imperative to act with reason. In Groundworks of the Metaphysics of Morals, Kant divides hypothetical imperatives into two subcategories: the rules of skill and the counsels of prudence.
Rules of skill
The rules of skill are conditional and are specific to each and every person to which the skill is mandated by. These are particular ends that we assign ourselves, and they provide a framework to understand how our ends can be achieved. Kant summarizes it as, "Whoever wills the end also wills the indispensably necessary means to it that is in his control." Kant's definition provides that there are a countless number of personal ends that can exist, because each human being has their unique perspectives, desires, personal circumstances, and intended methods to reach their ends. However, Kant also claims that there is at least one end that is universally sought after, and he determines that to be happiness.
Counsels of prudence
The counsels of prudence are attained a priori and have universal goals such as happiness. Counsels of prudence are actions committed for the overall sake of good will for the individual, and with the best intentions. This assumes, then, that actions done with the best intentions are using the hypothetical imperative to discern and make decisions that are "most moral good". Thus, almost any moral "rule" about how to act is hypothetical, because it assumes that your goal is to be moral, or to be happy, or to please God, etc.
Limitations
The general limitation of the hypothetical imperative is its potential ambiguity in its means, and its susceptibility to be misused for corrupted ends instead. Hypothetical imperatives also can only be acted upon if there is a personal investment in the action done and the ends produced. If one does not find personal benefit or incentive to conduct an action off a certain mean, then they are not obligated to do so. In other words, hypothetical imperatives invoke commands through "ought to do's", and their emphasis is more on individual personal desires. The only non-hypothetical imperatives are ones which tell you to do something no matter who you are or what you want, because the thing is good in itself. These types of imperatives belong to the category of categorical imperative.
