People who are formidable inspire fear and respect thanks to their size, or special ability, or unusual qualities. If you're a wildly popular celebrity, you probably don't go anywhere without a formidable bodyguard.
What's interesting about formidable is that we often tend to use it about things that are simultaneously a scary and impressive. A formidable opponent is almost by definition a worthy one; a formidable challenge is almost by definition one worth rising to. Not surprisingly, this word is derived from the Latin formidare, "to fear."