If you have a dual-SIM phone, you can have 2 SIMs in your phone, for example, one for 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.
Don't want work to interfere with your free time? Or do you have a cheaper data connection on one SIM? You can decide which SIM you want to use.