Repetition through difficulty. Each attempt strengthens the neural pathway until it finally clicks.
Extensive research shows that experience that causes repeated firing across a synapse "strengthens" it, with a key role played by the neurotransmitter glutamate.
Recall how an excitatory neurotransmitter binds to its receptor in the postsynaptic dendritic spine, causing a sodium channel to open; some sodium flows in, causing a blip of excitation, which then spreads. Glutamate signaling works in a fancier way that is essential to learning.
To simplify considerably, while dendritic spines typically contain only one type of receptor, those responsive to glutamate contain two.
The first (the "non-NMDA") works in a conventional way—for every little smidgen of glutamate binding to these receptors, a smidgen of sodium flows in, causing a smidgen of excitation.
The second (the "NMDA") works in a nonlinear, threshold manner. It is usually unresponsive to glutamate. It's not until the non-NMDA has been stimulated over and over by a long train of glutamate release, allowing enough sodium to flow in, that this activates the NMDA receptor. It suddenly responds to all that glutamate, opening its channels, allowing an explosion of excitation.
This is the essence of learning.
The lecturer says something, and it goes in one ear and out the other. The factoid is repeated; same thing. It's repeated enough times and—aha!—the lightbulb goes on and suddenly you get it.
At a synaptic level, the axon terminal having to repeatedly release glutamate is the lecturer droning on repetitively; the moment when the postsynaptic threshold is passed and the NMDA receptors first activate is the dendritic spine finally getting it.