What does this equation mean?

In 1971, Leon Chua published “Memristor — The Missing Circuit Element” in IEEE Transactions on Circuit Theory . His argument was one of pure symmetry, not empirical discovery. Circuit theory recognizes four fundamental variables — charge, current, voltage, and magnetic flux linkage — and, in 1971, three of the six possible pairwise relationships among them were already embodied in named components: the resistor relates voltage and current, the capacitor relates charge and voltage, the inductor relates flux linkage and current. Chua noticed that the sixth relationship, between charge and flux linkage, had no corresponding physical device, and reasoned from the completeness of the other five that one should exist. He named the missing element the memristor and gave it a defining relation: $M(q) = \frac{d\varphi}{dq}, \qquad v(t) = M\big(q(t)\big)\, i(t)$. where M(q) , the memristance, depends on the history of charge q that has flowed through the device — which is precisely what makes a memristor’s resistance a record of its own past, and precisely why it was recognized decades later as a natural candidate for an analogue, in-memory synaptic weight [ 3 ] .