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 can set this choice in Watch Settings, and the dwMap app on your watch will download it when you next start it. You can also change it and then refresh it during an activity by choosing Refresh on the app's menu (press-hold Up on watches without a menu button).
If you have Premium, you can also change this setting using the dwMap Companion Android/iPhone app. Note too with Premium that the background maps feature requires you use north-up, because the watch is not able to rotate the map images to follow your heading changes in track-up mode.