The physical address is calculated from 2 parts.
i) segment address.
ii) offset address.
The CS(code segment register) is used to address the code segment of the memory i.e a location in the memory where the code is stored. The IP(Instruction pointer) contains the offset within the code segment of the memory. Hence CS:IP is used to point to the location (i.e to calculate the physical address)of the code in the memory.