It's called Wear Mini Launcher, and if you have or are thinking about getting a Wear device, you'll definitely want to take note of it.
In short, Wear Mini Launcher gives you an easier way to access all the stuff on your Android Wear watch. By default, Wear lets you open any app via voice command -- by saying "start" or "open" followed by the app's name -- but if you want to actually scroll through the apps on your watch and select something with your fingers, it isn't so easy.
Wear does have an accessible menu of apps, but it's a bit buried in the system -- as is, perhaps more notably, the option to adjust the brightness on your Wear device. If you want to make your watch's screen brighter, you have to tap the watch's face, scroll down to the bottom of a long list to find the "Settings" option, tap that, then tap an option to adjust brightness and select from a set of predetermined levels. Not exactly something you're gonna do on the fly.
(The scrollable list of apps isn't quite as hard to find, but it still isn't convenient: To access it, you tap the watch's face, scroll down that same list to the "Start" option and then tap that to reach a menu of available applications.)
Wear Mini Launcher provides a more sensible path. Once the app is installed on your companion phone, you can simply swipe over from the upper-left corner of your smartwatch's face to open an easy-access app drawer. The drawer has links to all of your installed apps as well as a link to the device's settings and -- most significant, if you ask me -- a quick slider to adjust your watch's brightness.
Much better, right?
I wouldn't be surprised if something like this gets built into Wear itself eventually, but for now, Wear Mini Launcher is the best way to handle it -- and the tool is consequently a must-have for any Wear owner.