A Matter of Confidence or Issue of Confidence in the Parliament of the United Kingdom is a matter that is so important that the government of the day must demonstrate that it has the "confidence" of the House of Commons, namely that a majority of the House of Commons votes to support the government of day. If the government of the day is defeated on a confidence matter, by constitutional convention it will need to resign or call a general election.
The concept of a matter of confidence is broader than a formal motion of no confidence, which is a specific example of a matter of confidence. If the government is defeated on a motion of no confidence, that is a clear expression by the House of Commons that it does not support the government. However, other matters can also be matters of confidence.