Q: I don't understand the meaning of supplier... who is a supplier?
A: A supplier is a business or individual that supplies your business (you) with products or services that you use in your business.
For example, let's say you are a bakery. Some of your suppliers could be a flour milling company (supplying you with flour), the electricity company (supplies you electricity), a farming business (supplies you with various grains like wheat, corn, spelt, etc.). These are all suppliers.
Some accounting books, when they say "suppliers," specifically mean suppliers of raw materials (like grain, wood, raw minerals like copper, etc.). They are not referring to suppliers of services like electricity, water, repairs, etc. Just suppliers of products.