Android Market - developer console: marketing enabled / disabled?
The wording is pretty clear. It says:
"Do not promote my application except in Android Market and in any Google-owned online or mobile properties."
Meaning if you opt out (i. e. if the box is checked), entities other than Google (e. g. mobile carriers) won't showcase your app in their ads. I don't see why would anyone opt out, though.
To let them promote your app anywhere where possible, leave the box unchecked.