Modulation effects are a particular category of audio effect (FX).
These are effects which modify the sound in a certain way: by adding a time-delayed version of the sound to itself, and then varying the size of that delay over time.
Although this same process is common to all modulation effects, it can create a huge range of sounds! Audio effects ear training can let you learn to hear the different types of modulation effect, and even the parameters used to configure each one.
Some of the detail