The laser mode is the energy cross-section of the laser beam. The energy from the laser is not equal over the area that the laser hits, but rather can be in various distributions. There are four basic mode types: Gaussian, Doughnut, Multimode (combination of other modes), and single high order modes.
You can get an approximate representation of the mode of your laser by lasing some acrylic for a couple of seconds. This should cause an imprint in the general shape of the mode.
Most CO2 laser cutters have either a Gaussian or Doughnut mode.