Front back, meaning Neck and Bridge pickups. The Bridge side, is the one closer to your picking hand. The neck pickup is the one on the fretboard side. Both pickups produce different tones.
Simply put, if a riff sounds nice on Bridge pickup, use bridge position. If your licks sounds better in the neck position, use the neck position. The pickup selector switch (yeah that flicky thingy) selects the pickup positions.
'Up' is usually the neck pickup, 'down' position selects the bridge.
Strings vibrate differently in both the neck and bridge positions, therefore the 2 pickups help the player articulate these different tones in his playing. In the neck position, the strings vibrate wider than that in the bridge, therefore a rounder, looser tone is produced. In the bridge, there is less vibrations, therefore it has a brighter, tighter sound.
That said, I leave it to you to play with the positions. Explore it yourself, you'll understand better.