What does "wget -O" mean?

I have an wget command like this in my shell script:

reponse="`wget -O- http:localhost:8080/app/index.html`"

I don't understand the -O- option. I was explained that -O is output to somewhere and - is output to the current stream. I don't see any explaination of "-" in wget. Is that a standard thing for shell scripting. Where I can find reference to it?

Thanks,