A supplier (or subcontractors) are the companies which you use to supply your company with goods or services such as materials ie. sand, soil etc or people ie. laborers, operators etc.
Within the suppliers module, you'll be able to keep track of their key contacts, load up any supplier documents eg. Public Liability, Workers Compensation and also use tags to indicate specific account details eg. 30 day payment. Any expiry dates you have set for a supplier document will also be tracked and will flag in the top left of your dashboard.
If the suppliers module is not showing in your dashboard, just ask the support team to turn this module on for you. Once this is available, you will then also be able to rename this via Settings > Terminology if you'd rather it be called subcontractors or anything else.