Retrieval Practice (AKA The Testing Effect) is the single most powerful technique for building robust recallability of the concepts and facts that you want to remember. Whenever you retrieve something from memory, you increase your ability to retrieve it again in the future. You strengthen the synapses in the ENGRAM that stores that memory. An engram is the fundamental unit of memory storage and is comprised of a group of neurons who, through their synaptic connections retains the memory. Since neurons typically participate in multiple engrams, this scenario results in the capability for memories to overlap and explains why one memory may be associated with the next. If I say the word pizza to you, a series of memories may spontaneously come to mind, one associated with the next. Often the associations are not-obvious because they are simply the result of overlapping engrams.
Here engram A (gold) stores a memory related to a fish meal. Engram B (teal) stores a memory of some music that was playing during the meal. B shares some neurons in common with A. Thus, those memories are linked. When you think of the meal you might then recall the music and vice versa.
You can think of the retrieval of a particular memory akin to going to the gym and using a specific piece of equipment that isolates and builds one particular muscle. When you retrieve a memory, the associated synapses of its engram become stronger, perhaps enlarging or even adding additional synaptic connections. We have nearly unlimited storage capacity for memories. The problem with memory is that even if we store something, we easily forget how to retrieve it. Often the retrieval effort only gets it to the "tip of the tongue" and we aren't able to fully get it out. But, when we retrieve it, besides strengthening its engram, we create new pathways for future retrieval because we create new contexts that will help us retrieve it again. What do we mean by "new contexts"?
Each time you retrieve a memory, you do so in the context of what's happening and where you are when doing the retrieval. Maybe there is some other important event happening at the moment, or you're eating something delicious and memorable. There are tons of other sensory external events every second and internal events such as other thoughts you are having each moment. The retrieved memory may become associated with other engrams. The more cross-associations with other engrams, the more easily you will be able to find your way back when you need to. That's why, Robert Bjork, Distinguished Professor of Psychology at the UCLA says we need to focus on getting what we know out of our memory (retrieval practice) and spend a little less of our study time trying to cram more data in.