You can select which network modes (3G and/or GSM) your mobile phone can use. Your choice depends on the data speed you can achieve.
Tap the indicator next to Enable 3G. Depending on the current setting, the function is turned 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 3G and GSM provided you're within range of a 3G network. If you turn off 3G, you can't achieve high data speeds.