The matn is the actual text or content of a hadith, what the Prophet reportedly said or did, as opposed to the isnad, which is the chain of people who transmitted it. Even when a chain of narrators looks sound, scholars still examine the matn itself for problems that can reveal a hidden defect. They ask whether the wording contradicts the Quran or other clearly established, more strongly attested hadith; whether it conflicts with well-documented history or basic reason; whether it duplicates a report from a completely different, unrelated chain in a suspiciously identical way; and whether its language or theology sounds inconsistent with the Prophet's known style of speech. This mirrors the Quran's own principle that a genuine revelation from Allah will not contain internal contradiction: 'Then do they not reflect upon the Quran? If it had been from other than Allah, they would have found within it much contradiction.' Matn criticism (naqd al-matn) works alongside isnad criticism, not instead of it, because a good chain with a flawed text, or a flawed chain with sound-seeming text, can both mislead someone into wrongly attributing words to the Prophet.
Q&A · Hadith
What is the matn of a hadith, and how do scholars evaluate it for authenticity?
References
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.