David Ausubel
Ausubel was born in 1918 and grew up in Brooklyn, NY. He attended the University of Pennsylvania, taking the pre-medical course and majoring in Psychology.

Ausubel is most noted for his notion of the advance organiser. We can think of the advance organiser as simply a device or a mental learning aid to help us ‘get a grip’ on the new information. Put in more difficult language, according to Ausubel, the advance organiser is a means of preparing the learner’s cognitive structure for the learning experience about to take place. It is a device to activate the relevant schema or conceptual pattern so that new information would be more readily ‘subsumed’ into the learner’s existing cognitive structure or mental depiction!
Advance Organisers are simply devices used in the introduction of a topic which enable learners to orient themselves to the topic, so that they can locate where any particular bit of input fits in and how it links with what they already know. Ausubel's major principle — that the most important determinant of learning is what the learner already knows — calls for an image or example which directs the learner to relevant prior experience or learning and also points forward to new material.

Ausubel’s book explains his theory:

The Acquisition and Retention of Knowledge: A Cognitive View
Ausubel, D.P.

This is a college-level textbook that provides a comprehensive and credible theory of how humans can learn and retain substantial and growing bodies of potentially meaningful, organized subject-matter knowledge on an extended, long-term basis. It identifies explicitly the cognitive conditions under which such learning and retention occurs, and indicates how they are influenced by relevant cognitive structure, frequency, mental `set' and motivational variables, and, most importantly, by the probable underlying functional cognitive processes involved.

The other major contribution which Ausubel has made, is his emphasis on the active nature of reception or meaningful learning. The distinction between rote and meaningful learning is an important one, and too often we as educators fail to make reception learning as meaningful as possible. The need to require learners to be active by underlining, by completing missing words, by rewording sentences, or by giving additional examples, cannot be overemphasised in this context.

The processes of meaningful learning
Ausubel proposed four processes by which meaningful learning can occur:
Derivative subsumption. This describes the situation in which the new information you learn is an instance or example of a concept that you have already learned. So, let's suppose you have acquired a basic concept such as "tree". You know that a tree has a trunk, branches, green leaves, and may have some kind of fruit, and that, when fully grown is likely to be at least 12 feet tall. Now you learn about a kind of tree that you have never seen before, let's say a persimmon tree, that conforms to your previous understanding of tree. Your new knowledge of persimmon trees is attached to your concept of tree, without substantially altering that concept in any way. So, an Ausubelian would say that you had learned about persimmon trees through the process of derivative subsumption.
Correlative subsumption. Now, let's suppose you encounter a new kind of tree that has red leaves, rather than green. In order to accommodate this new information, you have to alter or extend your concept of tree to include the possibility of red leaves. You have learned about this new kind of tree through the process of correlative subsumption. In a sense, you might say that this is more "valuable" learning than that of derivative subsumption, since it enriches the higher-level concept.
Superordinate learning. Imagine that you were well acquainted with maples, oaks, apple trees, etc., but you did not know, until you were taught, that these were all examples of deciduous trees. In this case, you already knew a lot of examples of the concept, but you did not know the concept itself until it was taught to you. This is superordinate learning.
Combinatorial learning. The first three learning processes all involve new information that "attaches" to a hierarchy at a level that is either below or above previously acquired knowledge. Combinatorial learning is different; it describes a process by which the new idea is derived from another idea that is neither higher nor lower in the hierarchy, but at the same level (in a different, but related, "branch"). You could think of this as learning by analogy. For example, to teach someone about pollination in plants, you might relate it to previously acquired knowledge of how fish eggs are fertilized.