If you studied Brownian motion, you know that increasing temperature increases molecular motion. Similarly, decreasing temperature decreases motion.
Because we can imagine molecular motion increasing without limit, we put no upper limit on temperature. The situation with a lower temperature limit does not match because we can imagine a lower limit to molecular motion: no motion at all.
This experiment allows you to estimate the temperature at which pressure becomes zero. The pressure cannot go below zero. Thus, this temperature can be considered to be the lower limit for all temperature scales, an absolute bottom to any temperature scale. For that reason, scientists call it absolute zero.
The Procedure page contains more information about the experiment and how the process of extending a graph to values outside of experimental data (extrapolation) works.
You will be taking data for the pressure in pascals (Pa) and temperature in degrees Celsius. If your data fall onto a reasonably straight line, you will be able to extrapolate that line to zero pressure. Actually, the software will do this for you by providing the intercept of the pressure-temperature graph.