The Q-point is simply the DC operating point or DC biasing point for the transistor circuit. Typically, DC biasing conditions are set up to place the transistor and supporting circuitry in a condition that makes it useful, and no other signals (such as AC signals or swiching signals) are relevant when you consider the Q-point. Typcially, an AC signal or some other dynamic change (such as a switching event) is then applied. Sometimes the a small AC signal is applied and the circuit never deviates far from the bias point. However, if large dynamic changes are made, then the circuit variables change significantly from the Q-point.
So generally, with nonlinear circuits, with transistor circuits being a good example, we analyze the circuit first from the point of view of DC only and try to establish a Q-point. Then, we consider the dynamics around that condition. For small AC signals, we can create a linearized model of the system, and the linearization takes place at the conditions of the Q-point.
In the example you show, the Q-point is going to be at the intersection of the load line curve and the transistor curve for the Vgs condition you set up. The highlighted text is considering this fact and noting that if the Vgs is raised sufficiently that the transistor "resistance" is small compared to the load resistance, the actual value of Vgs does not significantly change the Q-point. You can see this in the figure which shows the dot on the load line on the left near the I_D axis. This is just a fancy way of saying that you turned the transistor fully on, and most of the supply voltage is on the load. Once the transistor is nearly fully on, driving it harder with large Vgs does not move the Q-point all that much.