PCB stands for "Potongan Cukai Bulanan" which is Malay for "Monthly Tax Deduction".
It is a series of monthly deductions that go towards payment of your taxes in relation to your employment income. These monthly deductions are retained by your employer and paid over to the Inland Revenue Board (LHDN).
This mechanism is designed to avoid the issues that come with requiring payment of a large sum at the end of the year.
PCB as final tax
Paying PCB can save you from having to file a tax report, if several conditions are met.
For more information see this article.