There are a few common ways to dim the pump.
The first is a potentiometer. This will possibly only have two contacts- both 'hot', one coming from the wire that used to go to the hot contact of the pump, and one going to the hot contact of the pump. The way the potentiometer works is by being a variable resistor which changes the voltage amplitude going to the pump. The smaller the voltage applied, the lesser the current through the inductor coils, and the smaller magnitude magnetic field. This lessens the displacement of the pump, letting it push less water, and thus reducing the pressure.
The second is a 'pulse width modulator' (PWM). This is powered by the wall, so it will have 4 contacts- what used to be the hot and neutral to and from the pump will now go to the PWM instead. The PWM has a hot and neutral of its own, which will now go to the pump. A PWM takes the AC signal and chops it up into smaller pulses. Instead of reducing the amplitude of the AC signal, it turns makes it choppy, reducing the total power. You can think of it as pushing the piston with several shorter pushes instead of a long push (original configuration) or a long but gentler push (potentiometer). The result is the same, the piston has less displacement, and the pressure is less.
The disadvantage to a potentiometer over a PWM in applications like, say, lighting, is that the potentiometer generates heat (that's where the energy goes that doesn't go to the thing down the line). It's wasteful. A PWM is more energy efficient.
In this application, this is a non-issue. There's only wasted energy while the pump is running, which is negligible power.
The PWM will have more inductive kickback than the original pump, which may decrease the lifespan of the pump. It doesn't immediately kill it, and I'm not sure how much of an issue it really is given that the pump likely has some mitigation for inductive kickback built in (because it would be a problem running as intended as well), as well as how short the pump runs for. In other words, the coils will get hotter than they normally would, which could cause them to fail sooner. I don't know how hot they get, but even if it does decrease the lifespan, the pump is $25. They last 5ish years already, if it decreases a year or two off it, I don't care.
I'm not an expert though, and there are some more complex and elegant options, but that's sort of how I understand it.