Most of the wines nowadays are produced in a conventional way, when farming involves the use of chemicals and artificial substances, industrial yeasts are added, corrective procedures are used, wines undergo sterile filtration and the addition of huge SO2 (chemical-based preservative) doses. All of this standardizes the wine and erases its identity. 

First of all, natural wine is made of grapes grown without any use of chemicals and artificial fertilizers. The transformation of grape juice into wine happens only with wild yeasts, all other stages proceed without any additives or corrections, and as little intervention as possible. Ideally, natural wines are not filtered and have no SO2 added, although very small additions of it are possible when there is a reason for that.