The light soldier is from the original game, The Conquerors. It is the first unit to be able to be seen in a game along with the construction soldier (water maps and territory mode only) and Command Center. In Conquest, Survival and King of the Hill game modes, during the start of time game you are given 3 light soldiers to work with.
In the beginning of the game, a light soldier is mostly used for claiming power plants or nuclear plants due to it being the only units spawned in the game. They do not do as much damage as a heavy soldier and aren't used much for raids. In later games, people use them less or not at all. It is recommended to use a mix of heavy soldiers and light soldiers as said in the help section in the game.
In the beginning game, it is not a good idea to raid people with even 10 light soldiers since they do minimum damage to anything. Later on as the game progresses, you could follow what the creator says and do a mix up with the soldiers to have a better attack.
Despite official recommendations, many people heavily recommend to NEVER use light soldiers, aside from using them to build plants during the beginning of the game. Instead, they recommend using only heavy soldiers. While heavy soldiers cost twice as much, they are still incredibly cheap and seem to be vastly more powerful. Even a mix of both light and heavy soldiers is criticized: a force of just heavy soldiers is said to be the best way to go. However, without knowing the actual stats (health, damage, etc.), it is difficult to properly make a judgement.The one exception where light soldiers are superior are when they're used as meat shields for explosive tanks.The light soldier is cheaper,has the speed to keep up with explosive tanks,and has the same amount of health compared to a heavy soldier.
They appear to be armed with M4 carbines, the assault weapon of choice for US infantry.