Modulation effects are those that change over time. Parameters of the effect are tied to a Low Frequency Oscillator (LFO).
If you don’t know what a Low Frequency Oscillator is, think of a clock and a light bulb. When the hand is on the 12 the light is all the way Off, as the hand moves past 1 the light begins to turn on; when the hand is on the 6 the light is all the way On and starts to turn off again as it moves past 7 back to Off at 12. This cycle repeats indefinitely, and you usually control how fast the clock spins. This is basically what is happening internally with each of these effects.