An oscilloscope measures two things:

  • Voltage
  • Time (and with time, often, frequency)

An electron beam is swept across a phosphorescent screen horizontally (X direction) at a known rate (perhaps one sweep per millisecond). An input signal is used to change the position of the beam in the Y direction. The trace left behind can be used to measure the voltage of the input signal (off the Y axis) and the duration or frequency can be read off the X axis.

Here are some interesting links: