Plain English Meaning
It's the part of learning that deals with how a student keeps hold of what they were taught and can bring it back when they need it. How well something is remembered depends on how well it was understood in the first place, how it was practiced, and how often it gets used again.
Full Definition
In aviation instruction, the phase of the learning process in which a student retains and can recall information, skills, or procedures that have been taught. Memory in this context is generally described in three stages: sensory register (very brief intake of stimuli), short-term or working memory (where information is held and processed for a limited time), and long-term memory (where meaningful, well-organized information is stored for later recall). Effective remembering depends on how well the material was understood, organized, and rehearsed during initial learning, and on how often it is used or reviewed afterward.