What does innocent until proven guilty actually mean?

The presumption of innocence is a procedural allocation of burden. In a criminal case the government must prove every element of the offense beyond a reasonable doubt, and the defendant is not required to testify, present evidence, or prove innocence. Jurors are instructed that the presumption stays with the accused throughout trial and is overcome only by that standard of proof. The Supreme Court has treated it as a component of due process. Reasonable doubt is deliberately not quantified into a percentage, and courts generally resist attempts to define it numerically, describing it instead as the kind of doubt that would cause a reasonable person to hesitate in an important decision.

Two clarifications resolve most confusion about it. First, the presumption governs the courtroom, not public opinion, journalism, employment decisions, or your own judgment, and saying someone is presumed innocent is not a claim that they did not do it. Second, it applies to criminal proceedings. Civil cases use a much lower standard, preponderance of the evidence, meaning more likely than not, which is exactly how a defendant can be acquitted criminally and still lose a civil suit over the same conduct. Related but distinct: the burden shifts for affirmative defenses like self-defense or insanity, where the defendant may have to produce evidence, and pretrial detention does not violate the presumption because it addresses flight risk and danger rather than guilt.