What is hypertonia? (in short understandable terms)

More "stiffness": Imagine if your arm was a pocketknife. That increased "push" you need to do to get the knife out is similar to increased tone. A doctor checks your tone byl passively move your arm. If there is increased "stiffness", this is increased tone. This occurs in diseases such as multiple sclerosis, stroke. If it is "floppy", this is decreased tone, which happened in polio and in guillain-barre syndrome.

I'll try : Hypertonia implies that the muscles are "set" at a higher baseline level of tone; even at rest. The muscles are incapable of completely relaxing. This is not a good term for doctors to use in isolation, because it doesn't give patients a lot of information. It is almost always a symptom, and not a complete diagnosis. Hope this helps.