The router broadcasts two WiFi signals, one for the 2.4GHz band, one for the 5.0GHz band. Your device can connect to either one, and you can connect to the 5G sometimes (for example, when you are near the router), 2G other times (for example, when you are further away from the router and the 5G signal is weak).
The router is configured to have different SSIDs for the two bands, so that you can choose which band to connect to. Alternatively, you can give the same SSID to both bands on the router. In that case, your will see only one SSID and your device will choose which band to connect to.