What does the specification 4-8 Ohm mean?

The indication "4-8 ohms impedance" refers to the nominal resistance that the speaker has.

The impedance of a speaker refers to the electrical resistance it offers to the amplifier.
The rated impedance of a speaker indicates how much resistance it has when driven by a particular signal.

A speaker with a lower resistance can draw more current and therefore more power from an amplifier.

It is important to note that the impedance of a speaker is not always the actual resistance that can be measured with a meter on the speaker. The resistance of a speaker varies with frequency, and a speaker with an impedance of 4 ohms may actually have a resistance range of 3 to 20 ohms.