The GDT (Global Descriptor Table), LDTs (Local Descriptor Tables) and the IDT (Interrupt Descriptor Table) , all located in memory provide protected access to system resources. The GDT and IDT are pointed to by special registers within the CPU (so the processor can access them), and the GDT entries point both to the operating system segments and to the LDTs. The IDT contains pointers to code that needs to be executed when interrupts occur.
Each running program has its own LDT but there is a single GDT for the entire system.
The LDT describes segments local to each program including its text (code), data, stack and so on. The GDT describes system segments including the operating system itself. (Segments are independent address spaces. Segmentation is one of the techniques use to manage memory on computers. More on segments when we discuss memory management but for now just understand that with segmented memory, we need a two part address to reach any memory location - the first part is the address of the segment and the second part is the location (offset) within the segment.)
So the GDT basically points to (i.e. contains the physical addresses of) segments that contain the operating system code and data but also provides a way to get to all the LDTs in the system.