The CIS is a HMRC scheme which applies if you work for a contractor in the construction industry but not as an employee, so for example as a self-employed individual. The CIS rules mean that the contractor is usually obliged to withhold tax on its payments to you, at either 20% if you are 'registered' or 30% if you are not.
If you are engaging other self-employed workers doing construction work then you are a contractor and must register under the CIS. There is more information on how to register for the CIS as a contractor on GOV.UK. Subcontractor.
As a self-employed worker, it is more likely that you will be engaged, or taken on, by a contractor to perform some construction work. This will mean you are a subcontractor. If you are a subcontractor, then you can choose whether or not to register under the CIS.
You can apply for gross payment status when you register for CIS . This means contractors will be paid in full, without deductions. Tax and National Insurance at the end of the tax year.