A "scheme" is MetaMeta's general term for a codeset, lookup table, metadata standard, data dictionary, and so on. Some schemes are super official, managed by standards bodies and with changes hotly debated. Others are based on simple convention or historical accident.
Some standards are versioned, with updates on a yearly or more infrequent basis. For example, the ICD10 is refreshed yearly, with codes being added, removed, and refined. In these cases, a scheme refers to a single version or vintage, with the version indicated in the Scheme ID.