I'm fairly new to these games - not sure I understand what "Hordes" means

At it's "purest" it means the continent bonus structure is such that you get a +1 bonus for owning a territory and all the territories that border it.

Some boards combine that with other bonus structures. Looking at your games list I see that you are in "North America" by Kjeld (one of my favorite boards, but is far from straight-forward). In that board you get a +1 for owning a territory and all the territories that border it within that country (Canada, USA or Mexico) disregarding the ships.

The best way to see what territories make up what bonuses is to click on the icon in the upper right bar that has 3 little blue boxes and some grey lines. That pulls up a list of all the continents. Click on one of those and the territories that make up that continent get highlighted. 

As you run into other terms, you can also check the glossary on the help wiki:

http://www.wargear.net/wiki/doku.php?id=wargear_glossary

Which has an entry for hordes that points to this intimidating and exhaustive description:

http://www.wargear.net/wiki/doku.php?id=designer_tutorials:tutorials:other_continents:other_continents#hordes