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. 

How to care your vegetable-tanned Leather?

Leather is a living material that ages and becomes more beautiful, provided it
is regularly maintained and cared for in accordance with its nature. To keep your leather products looking their best, it's important to follow a few simple care tips. 
Use a leather care product specially
formulated for vegetable-tanned leather (a mixture of vegetable oil and
beeswax) and apply it with a clean, dry cloth.

Allow to dry, then rub off any residue with a soft cloth. This will moisturize the leather and prevent it from drying out and cracking.
Avoid harsh chemicals and exposure to water. Vegetable tanning Leather is sensitive to aggressive chemicals and water.