Soldiers are lance-wielding infantry units which, for the most part of the Fire Emblem series' history, are unplayable and unpromotable. They usually have abysimal stats compared to other units at the same level, give you less experience than other types of enemies, and stop appearing later in the game. They are a playable class only in Fire Emblem Gaiden and in the Tellius series games. They were also an unused class in Fire Emblem: Seisen no Keifu.
In Gaiden, they're the first-tier class of the Knight/Baron path.