what actually shuts down when you go on cycle and why does it matter before your first pin?

been doing a lot of reading before my first cycle and keep seeing people talk about being shut down and HPTA and LH and FSH. can someone explain this in plain english? i get that your natural test production stops but why exactly does that happen and what does it mean practically? like why do you need PCT and why do people run HCG? just want to understand what is actually happening before i start so i am not just following a protocol without knowing why

this is the explanation i needed. so the pituitary stops sending LH and FSH signals because it thinks test is already high enough from the exogenous test, and then the testes stop producing. that is what shut down means. and PCT with nolvadex or clomid blocks estrogen at the pituitary which makes it start producing LH and FSH again and restart natural production. makes sense now. and having PCT ready before first pin, not ordering it after the cycle - noted.

The HCG explanation makes the PCT rationale make much more sense to me. The testes stop producing because the pituitary stops sending the signal. HCG mimics LH and keeps the testes partially active during cycle. So when PCT starts the testes have not fully atrophied and the restart is faster. Going to add HCG to my protocol for the next cycle at 500 IU twice a week throughout. The PCT drugs I had sorted before starting already - first good decision I made on this stuff.