Data governance is the exercise of authority and control over the management of data. It does not manage data itself; it decides who does, what they are allowed to decide, which rules they must follow, and who answers when something goes wrong. Everything else people call governance — a catalog, a quality dashboard, a stewards' forum — is either an instrument for making those decisions or evidence that they were made.
In practice. It looks like a short list of named people with named decisions attached to them. Who approves a change to the definition of "active customer". Who signs off that a dataset may leave the finance domain. Who is called when a report and a system disagree. When a governance program is working, that list is written down, current, and known to the people it constrains.
Where it goes wrong. Governance is bought as a tool and staffed as a committee, and neither creates a decision right. A catalog with no owner recorded against a term is a directory; a council with no authority is a meeting. If nobody can point to a decision the program has actually made in the last quarter, there is no governance yet — only its documentation.