This is the most important distinction in BaZi.
Ming (命) = your natal chart, your fixed blueprint. The eight characters you were born with. Your Day Master, your element balance, your 10 Gods configuration. It doesn't change.
Yun (运) = your Luck Cycles, your movement through time. The 10-year cycles and annual stars that interact with your Ming. It is always shifting.
Ming is your hardware. Yun is the software updates that arrive every decade. A person with a "weak" Ming who gets a 30-year Useful God Yun can live an extraordinary life. A person with a "strong" Ming who enters a draining Yun may struggle despite their gifts.
When people say "fate," they usually mean Ming. But BaZi wisdom says: Ming is 30%. Yun is 30%. Your choices and environment are 40%. Never confuse the map with the journey.