Not all aspects of a particle system can be defined in advance. Sometimes, certain parts of a particle system's behavior will need to be controlled or changed at runtime. For example, you may be creating a magic effect that changes color based on the amount of magical energy consumed when the spell is cast. In such cases, parameters will need to be added to the particle system.
A parameter is a type of property that can send or receive data to/from other systems such as Blueprints, Matinee, a material, or many other sources. In Cascade, just about any given property can be assigned to a parameter, which means that the property can then be controlled from outside the particle system.
For instance, setting up a parameter to control spawn rate for a fire effect could allow for the player to have the ability to increase or decrease the amount of flame being emitted at runtime.
Conversely, there are parameter modules which can be added to a particle system that can, in turn, drive other things in the level such as a color used in a given material.
In Cascade, parameters are generally established by way of Distributions, which are ways to handle data within a property. You can find a general overview of distributions below, or you may read more detailed information about them on the Distributions page.