The signal to be observed is applied to the Y plates.
It is attenuated if it is too large in amplitude.
It is amplified if it is too small in amplitude.

The timebase generates a sawtooth. (see page on waveforms).
The sawtooth is variable in frequency and amplitude.
As the sawtooth voltage rises, it causes the spot to sweep from left to right, across the face of the CRT.
When the sawtooth suddenly falls to zero, the spot flies back to the left of the screen ready for another sweep.
This sweep and flyback usually happens so quickly that that the display appears as a straight line.

So we have the spot being deflected horizontally by the timebase, while the signal is deflecting it vertically, by means of the Y plates.
The combination of forces produces a display which represents the input signal.

To display one cycle, the timebase and signal frequencies must be the same.
If the frequencies are slightly different, the display will drift sideways.
To avoid this , the input signal is sampled by the trigger circuit and used to ensure that the timebase runs at exactly the same frequency as the input signal.