Every parameter has a type - a circle's radius is a Real number, its color is 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, the values of parameters can be changed, either by directly manipulating the item on the canvas, or by using the Parameters Panel window to alter the values by typing them in directly.
Some parameters can be defined has statics : they will not accept changes over the time.
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.