CIS stands for "Construction Industry Scheme". It is a compulsory scheme for individuals, businesses and companies working as contractors and subcontractors in the construction industry and collects tax due on earnings during the year.
If CIS applies to you then you have to register with HMRC. If you are a subcontractor you will usually have tax deducted from what you earn by the person you are working for. At the end of the year you will need to do a tax return to pay any extra tax you owe or to reclaim any overpaid tax.
If you are a contractor then you will have to deduct tax from your subcontractors and pay it over to HMRC each month.
It is possible to be both a contractor and subcontractor at the same time. It is also possible to register for what is called "Gross Status", meaning tax is not deducted during the year, but you still have to report monthly and comply with the CIS rules.
I can help you decide if the rules apply to you, help you register and help you run CIS.