When you do something silently, you make no noise at all. If every guest at your dad's surprise party stays silently hidden until he turns on the lights, he'll definitely be surprised.
The adverb silently is perfect for describing actions without sound or words. You can silently implore your friend to help save you from an awkward conversation at a party, using only your facial expressions. Or you can silently sneak out of an extremely boring lecture, tiptoeing toward the exit. If your annoyed sibling tells you to eat silently, you can comply by silently chewing your sandwich — or rebel by noisily crunching potato chips.