The low-gain antenna sends and receives information in every direction; that is, it is "omni-directional." The antenna transmits radio waves at a low rate to the Deep Space Network (DSN)(see communication) antennas on Earth.
The high-gain antenna can send a "beam" of information in a specific direction and it is steerable, so the antenna can move to point itself directly to any antenna on Earth. The benefit of having a steerable antenna is that the entire rover doesn't necessarily have to change positions to talk to Earth.
The rovers not only communicate with earth but also communicate with the two spacecrafts orbiting earth, that is the 2001 Mars Odyssey and Mars Global Surveyor. The orbiters have much more energy than the rovers and also have a more powerful antenna. By sending the signals to earth, via the orbiters, the rovers save a lot of energy. Also the orbiters are closer to the rovers than the Deep Space Network antennas on Earth.Some times, the rovers are on that side of mars which does not face the earth. But the orbiters have Earth in their field of view for much longer time periods than the rovers on the ground.