To expand.

Voltage is a relative measurement - think of it as "what is the difference in electrical pressure between these two points".

If you measured two ends of the same wire, no matter what else is happening on that wire, voltage will read close to 0, unless the wire is extremely long (150 ft or longer). Most of the time when we say "oh that's a hot. It's 120v" we're using a reference that is always constant - the ground. Like the literal ground beneath your feet (or the ground wire, which eventually goes into the actual earth outside your home). Because that measurement tells us how likely (and painful) it's gonna be to get a shock from it. More volts = more pressure = more likely and more pain.

When the switch closes (aka in the "on" position), it's the same as having one long chunk of wire rather than two chunk, when it's open (aka in the off position), it's separated agian. So when it's open there is a difference in electrical pressure - one side has 120v (referenced to ground) and the other side has 0v (agian, as referenced to ground) as they are now two separate wires, one with a connection to the panel, the other with a connection to the lights.