The best answer for this is to say that tenantry and sharecropping led to a situation in which African Americans (as well as many poor white farmers) were stuck in a system in which they were almost completely under the thumb of the landowner on whose land they were working.
In this system, sharecroppers would essentially have to borrow money from the landowner so that they could buy or rent things like tools or seeds. They would also, of course, have to give part of their crop to the landowner. What this meant was that the sharecroppers often ended up getting exploited. They had to borrow from the landowner and so the landowner would impose unfair contracts on them. This would cause them to have to do things like paying exorbitant amounts for the things they needed. When they did this, they would go so deeply into debt that they could essentially never get out. Once this happened, they were completely at the mercy of the landowner.
Thus, tenantry and sharecropping ended up leading to a very bad economic situation for those who had to participate in it.