A polyprotein (which etymologically means "several proteins") is the product of a single gene as a single protein which is usually non-functional.
The polyprotein is then cleaved at specific sites into smaller proteins by one or more proteases. When they are separated from each other the proteins can then perform their function.
This synthesis strategy for proteins is used by different viruses (Hepatitis C, Poliovirus...)
The Poliovirus polyprotein is firstly cleaved by protease 2A, and then by protease 3C, leading to around ten smaller proteins. © UCL / Licence Creative Commons