Common law is a body of law built up over time through judicial decisions, rather than being set out entirely in advance through statutes passed by a legislature. Judges look to earlier rulings on similar facts to guide how a current case should be decided, even when the stakes seem relatively low.
This reliance on precedent gives common law systems a degree of flexibility, since the law can adapt gradually as new situations arise, while still offering predictability, because a well established line of earlier decisions signals fairly clearly how a new dispute is likely to be resolved, a nuance that shapes how it plays out.
Common law operates alongside statute law rather than replacing it, and legislatures can override or modify judicial precedent by passing new laws, meaning the two sources of law continually interact rather than existing in separate, sealed compartments, particularly when scrutiny is otherwise limited, which few people stop to consider.