Leather tanning is the process of transforming hides into leather. This operation transforms a putrescible hide into a rot-proof material: leather.
There are 2 leather tanning methods: vegetable tanning and mineral tanning.

Vegetable tanning is a traditional method that has been used for centuries.
Vegetable tanning requires the use of natural vegetable tannins, such as plant extracts or powdered bark from quebracho, mimosa or chestnut trees.
One of the main advantages of vegetable-tanned leather is that it is a natural product, free from harmful chemicals. The tannins used in the vegetable tanning process are environmentally friendly and biodegradable.

Another advantage of vegetable-tanned leather is its natural look and unique feel, provided the leather is neither pigmented nor plasticized. Natural tannins give leather a unique patina that develops over time, making every item made with vegetable-tanned leather unique. Patina is a natural change that occurs on the surface of leather over time. As leather is exposed to the elements and daily use, it develops a unique patina, unique to each piece of leather.
(source : Rademecker)

Vegetable leather requires more regular maintenance to maintain its color and suppleness than other leathers.