You can select which network modes your mobile phone can use, eg. 3G or GSM. Your choice determines the data speeds you can achieve.
Tap the indicator next to "Enable 3G" to turn the function on or off.
3G enables you to achieve higher data speeds than when using GSM. If you turn on 3G, your mobile phone automatically switches between the two network modes provided you're within range of a 3G network. If you turn off 3G, you can't achieve high data speeds.