A sharecropper lives on someone else's farmland and pays, as the rent, a large share of the crop that comes from this farmland. Sharecroppers were, for this reason, poor; it was nearly impossible to clear up the initial debt incurred by renting someone else's land and farming it for profit, the bulk of which went to the landowner.
In Things Fall Apart, by Chinua Achebe, Okonkwo goes to a wealthy man to ask for seed yams to start a crop:
Sharecropping was a very slow way of building up a barn of one's own. After all the toil one only got a third of the harvest. But for a young man whose father had no yams [to pass on to his son], there was no other way.