What is a tithe barn why did Medieval England use a tithe barn and what happened in a tithe barn?

The word "tithe" is from Old English teoða, which means a tenth.

A tithe was a tax of one-tenth of all produce or income imposed by the Church (not the government, as has been incorrectly claimed in another answer) on everyone in each parish, so farming peasants were forced to give a tenth of everything they grew to their local priest. This produce would go straight into a community tithe barn, where it would be stored for a time; in most cases it would then be used to feed Church employees and the priest himself, or it could go to a local monastery, or it might be sold at market rates and the money would go into Church coffers.

The tithe was universally hated since it imposed an additional unwelcome burden on the poorest people in society, who were also bound to pay money taxes, rents, fines and other obligatory payments to their feudal lord.

In a particularly bad season when the crop was poor or was damaged by insects, weather or disease, the tithe might be returned as a charitable donation to the farming peasants, but it would be a meagre food supply in such cases and starvation was a real threat.