Rooting is pretty much opening up the devices OS so that you have complete control over whats on it. If there are system applications that you don't use but are just taking up space, rooting the device will let you delete those applications. Rooting also lets you install custom Roms (if they are available). Pretty much you have total control over your device and its not locked down.
Quite simply, you download and install an app called gApps4Archos.apk or gAppsInstaller_v4.apk depending on Firmware or Android version. Search and research. It is NOT that hard.