What does it mean when there are two rows of signal bars on my iPhone?

Two rows of signal bars indicate your iPhone is a Dual SIM model. The upper row shows the signal strength of the line used for cellular data, while the lower row shows the signal strength of your other line. If there is no signal, 'No Service' appears. You can open Control Center to see the status icons with their corresponding cellular plan labels and carrier names. (Page 1)