Everything written about the druids in their own time was written by outsiders, most of them Greeks and Romans and several of them at war with the people they were describing.
Julius Caesar gives the fullest account. He says the druids were one of two honoured classes in Gaul, that they judged disputes, controlled sacrifice, were exempt from tax and military service, trained pupils for as long as twenty years, refused to commit their teaching to writing although they used Greek letters for other purposes, taught that souls pass from one body to another, and presided at human sacrifices including the burning of victims inside a giant wicker figure. Pliny the Elder describes a white-robed druid cutting mistletoe from an oak with a golden sickle. Strabo, Diodorus and Lucan add divination read from the death throes of victims.
These reports are not disinterested. Caesar was justifying a war of conquest to a Roman readership, and later imperial policy treated druidry as a political threat to be suppressed. Much of the material may descend at second hand from Posidonius, and parts of it repeat standing Greek and Roman tropes about barbarians in general. Against that, archaeology has produced evidence consistent with ritual killing: the Lindow bog body from Cheshire, killed by several methods at once, and the weapon and bone deposits at the Gaulish sanctuaries.
Scholars therefore divide. Some treat the classical accounts as broadly usable where they agree with each other and with the ground; others treat them as literature about barbarians that happens to be set in Gaul. The defensible position is that druids existed and held a substantial social role, and that the content of what they taught cannot now be recovered.