Simplistic summary :
Filter envelope = timbre adjustments,
Amplitude envelope = volume adjustments.
A few more thoughts :
Envelopes differ from one synth to another, especially from one synth manufacturer to another. Many older synths had rather limited envelopes, as each part (for example, in a simple ADSR) would not necessarily give you distinct controls for both the level and time of it. In other words, A or attack is not just a time, it is also a level. In a great machine like the Kronos, all envelope levels and times can have distinct values, so you have much more control to adjust things as you want.