I know a fair bit but I could use a primer.
The Han dynasty had been becoming increasingly corrupt over the years, the rich gentry families accumulated power and could evade taxes while the commoners coudn't withstand the huge taxes and corvee labour imposed on them. There were also several child emperors which is not good for stability. A huge peasant rebellion happened because of this (yellow turbans), the rebellion was supressed succesfully but it basically killed the dynasty.
Regional lords were appointed afterwards which had both administrative and military power over huge regions, previously, the commandery was the biggest effective administrative unit, and the commandery administrator could not mobilize troops outside their commandery to prevent rebellions. The new guys had control over several commanderies. Well, you can imagine what happened next, right?