Do judges have to follow what earlier judges decided?

In common law systems, largely yes, but the rule is more flexible than it sounds. Precedent is a past judicial decision that serves as authority in later, similar cases. Under the principle of stare decisis, "to stand by things decided", past decisions guide future rulings, promoting consistency and predictability.

The binding part has layers. Vertical stare decisis binds lower courts to follow the decisions of higher courts in the same jurisdiction; horizontal stare decisis is the principle that a court adheres to its own previous rulings. And only part of an earlier opinion binds. Its ratio decidendi, the line of reasoning that forms the basis of the judgment, creates binding precedent, while obiter dicta, comments made in passing that were not necessary to resolve the case, may be cited as persuasive but do not bind.

Judges also have ways out. A court may "distinguish" a precedent, declining to apply it because the facts of the new case differ materially, or it may overrule it, when the same or a higher court finds its principles erroneous in law or overtaken by new legislation or developments.

Not everyone lives under this regime. Civil law systems do not usually use stare decisis, since it would violate the principle that only the legislature may make law. They rely instead on jurisprudence constante: a consistent line of well-reasoned decisions is highly persuasive but not controlling.