We call these "thermometers" because of their shape, but they're not related to temperature.
Drag either of the thermometers to change its length.
If you want to record different states of the image, clicking the Graph button will produce a graph in another window, which will allow you to plot points and connect them.
The drop down list allows you to choose scenario 1, 2, or 3.
The Reset button will clear everything and start over.