Thermo weapons rely on thermal fusion to damage enemies. This can be either drawn naturally from stars, or made artificially in a lab.
Thermo weapons tracked automatically in GoF1, but in GoF2, they require a lock on before tracking, otherwise they spout in random directions, which is incapable of hitting targets, save at very close range.
Thermo weapons are best to be used at close range in both GoF1 and GoF2. In GoF1, the reason for this is that the game engine cannot draw very many particles at a time, so getting bolts to connect quickly is the key because when the particles connect, the game engine is able to fire more. Hence, if you are closer, the weapon connects faster, and fires again faster, increasing your rate of fire dramatically. However, in GoF2, the player is advised to track targets to get lock-on at long range, then to get up close to avoid the same problem.