Robot mode is the form that Transformers have outside of their alternate mode. Although generally a bipedal humanoid in shape, the robot mode is not always so. Prominent examples include the Mini-Cassettes with animal based forms, as the cassette would be considered their alternate mode, while the "beast mode" would be considered their robot mode. Another unique situation concerns the Mutants, Transformers who have lost their robot mode but acquired two beast modes.
Generally speaking robot mode provides greater freedom of movement and better combat capabilities (though this can also be said of alternate modes which are designed for combat). Often, the alternate mode will be given or taken to better facilitate the robot mode, such as repair (which is what Teletraan I did with the Autobots and Decepticons aboard the Ark) or for protecting the robot mode (which was done to protect the sparks of the Maximals and Predacons during the Beast Wars). After taking an alternate mode, the robot mode will display characteristics of the alternate mode.