That.
Or explained in another way:
It shows the change of voltage over time - to examine very fast voltage changes. Any other value, not only electrical, can also be measured. But it must be represented as a voltage, and the rate - how many V represent x A, N, Pa,... whatever - must be known, of course. Oscilloscopes can only measure voltage. So any other value must be transformed into voltage first.
In default mode the abscissa ("X"-axis) represents the time (depending on the scale adjusted tiny fractions of a second), while the ordinate ("Y"-axis) shows the voltage in volt, millivolt... - it's [mostly auto]adjustable, and depends on the scope.
Is it a periodical signal, the changes repeat the same way all the time (sine wave, saw tooth, rectangle [PWM or binary transmissions up to some MHz], scan and display are synchronized to the signal's frequency, so you can watch and measure within the signal as it was standing still.
You can also record a timespan of a signal, and then look at it, if you want to take a look at a voltage for quite a while, and see if it changes (depending on the scope long term measurements over hours or days are possible, but better use a so called logger for that.) or some single incident, like the voltage during a spark:
That's simplified the basics. Oscilloscopes are very comprehensive multi-measurment devices. Today's oscilloscopes are almost all digital, which means they are embedded computers, bringing a lot of analyzing software like FFT, networkanalyzers and other things.
It has many purposes, like to see how much noise a signal has, if it contains tolerable peeks, etc. etc. - to see, if a circuit does it's job as intended to resp. clean enough, and if not, what may cause the problem. An experienced electronics technician/engineer can trace back certain "patterns" to certain causes.
But unless you start on electronics yourself, you don't need to learn more.
