A seismometer records the vibrations from earthquakes. Mechanical versions work by way of a large mass, freely suspended.
In the example on the left, a rotating drum records a red line on a sheet of paper. If the earth moves (in this case from left to right) the whole machine will vibrate too.
However, the large mass tends to stay still, so the drum shakes beneath the pen, recording a squiggle!
The confiner prevents the mass from bouncing around all over the place.
Incidentally, a seismograph is the graph that a seismometer draws - don't get these muddled up!