Before a segment can be used, the contents of the descriptor for that segment must be loaded into the segment register. The 386 has a complex set of criteria regarding access to segments so you can't simply load a descriptor into a segment register. Also these segment registers have programmer invisible portions. The programmer loads one of these registers with a 16-bit value called a selector. The selector uniquely identifies a segment descriptor in one of the tables.
Access is validated and the corresponding descriptor loaded by the hardware.
It is biased towards the paging hardware and the associated page level protection.