As you check filters, the system will show heroes that fit within the requirements of those filters. The more filters that are selected, the more lenient the system will be for finding a hero within those filters. For instance, if "Jungle" is selected, Predator will not appear because only the strongest jungle heroes will be available. However if "Carry" is also selected, Predator will appear as an option because he can carry as well as jungle even though he cannot jungle as effectively as other heroes.
Two really neat features of this system are the Favorites Filter and Filtered Random. Favorites allows you to go into the herodex and set your "favorite" heroes. The Favorites Filter will then be available in the hero pick screen and you may sort through heroes with this filter as well as others. Filtered Random is just like it sounds: when you have any filters selected and you random a hero, you will random from the heroes within your current filters.
I'd like to give thanks to all of the community feedback that drove the creation of this feature! If you have any suggestions or feedback on the hero filters or HoN in general, please visit the suggestions forums and make a post about it.