What does it mean to be "catatonic" (in english please!)?

just wondering, in a book i was reading (new moon, actually) they mention the doctor possibly diagnosing Bella as "catatonic"??? That was used in another book I was reading (the host, actually). They send Wanderer was practically catatonic. WHAT DOES THIS MEAN?! In English! I searched it on google but I didn't understand the definition. Please help! Oh and what happens if you are "catatonic"?