North-up means the map display on the watch matches the display of online maps, with North always being "up" on the watch screen. The position/heading arrow points in the current direction of your travel on the map.
Track-up rotates the map on the watch as your heading changes so that "up" on the screen matches your forward direction of travel. This matches the way that many vehicle GPS units work, showing something of a "first person" view of what is ahead.
You set this choice in Settings on the website, and the dwMap app on your watch will download it when you next start it. You can also change it on the website and then refresh it during an activity by choosing Refresh on the app's menu (press-hold Up on watches without a menu button). Learn more about the dwMap menu.
Note with dwMap Premium, the background maps feature can only draw background maps in north-up orientation. If you normally use track-up mode, it will automatically switch to north-up mode when background maps are enabled, and back to track-up mode when the maps are switched off.