If you apply a voltage with unlimited current, the leds of the 7 segment display will get overheated very quickly if the voltage is too high.
If you apply a current, there a a voltage over the leds. But that voltage can be called a 'voltage drop'.
Use any led you have, and try a few things. If you connect a led to a 5V power supply without resistor, you might see a flash or not. But the led is broken. If the voltage is too low, the led doesn't do anything. Connect the led with a resistor to 5V and it is okay. Perhaps you should wear safety glasses.