As a noun, it refers to the sound produced when someone strikes something.
* Someone knocked on the door.
As a verb, it means to strike something, especially with a loud noise.
* He knocked on the door loudly.
* Someone knocked on my window last night.
Additionally, "knock" can also be used in figurative contexts, such as:
* He's been knocking on my door for weeks, trying to get me to invest in his business. (In this context, "knocking on someone's door" means persistently requesting or asking for something.)
* I have a few ideas, but they're just a shot in the dark. I'm throwing them out there, but I won't be surprised if they don't knock anyone's socks off. (Here, "knock someone's socks off" is used in an idiomatic sense to express the idea of impressing someone greatly or making a very positive impression.)