If you have a dual SIM variant, you can have 2 SIMs in your phone, for example, one for your work and one for your personal use.
When, for example, making a call, you can choose which SIM to use by tapping the corresponding SIM 1 or SIM 2 button after you dial the number.
Your phone shows the network status for both SIMs separately. Both SIM cards are available at the same time when the device is not being used, but while one SIM card is active, when, for example, making a call, the other may be unavailable.
Tap the SIM you want to rename, and type in the name you want.