Incremental resistance is, as others pointed out, the slope at a particular point. The important thing to understand is that, since the slope of the curve changes depending on the point of the I vs V curve you are located, the resistance changes as well.
Take for instance an ideal resistor, if you change the signal voltage you apply to it the resistance doesn't change, but, with transistors or diodes, resistance is dependent on the signal voltage applied to them. So the main application of the theory behind all of this is that if the signal voltage variation is small enough, the resistance variation is also small and we can approximate it as a constant resistance. This is the so-called small signal model or small signal approximation.