A ValueNode is a value that changes over time.
An example of a ValueNode would be:
"3.4, for ever"
A more complex example: "3.4 at the beginning of the animation,
move smoothly up to 7.6 at time = 10 seconds, then jump instantly to 2.0 and stay there until the end of time"
These are both descriptions of a value (in this case a real number) and how it changes (or doesn't!) over time.
ValueNodes are the things we see as the value for every parameter of every layer. The waypoints visible in the timetrack dialog are also part of the ValueNode - waypoints are how ValueNodes work out what value to be at each point in time.