Parameters are properties a layer or may have, such as "Blend Method", or for a circle its "Radius".
Every parameter has a type - a circle's radius is always a real number, its color is always a color, and so on. Synfig Studio won't let you set its radius to Red, or its color to 5.7, because it knows what type each parameter is supposed to be.
In Synfig, these can all be changed, either by directly manipulating the item on the canvas, or by using the Parameters Dialog window to alter the values by typing them in directly.
Parameter values are ValueNodes. This basically means that their value can be animated over time. They can be simple:
"this radius is 3.4, and stays 3.4 for ever"
but are a complete description of the value through the course of the animation
"the radius is 3.4 at the beginning, moves smoothly up to 7.6 at the middle, then jumps instantly to 2.0 and stays there until the end of time"
This complete description is achieved by the use of waypoints, which are associated with each ValueNode.