Mitcham definition from https://www.gotquestions.org who provide Bible definitions.

No one is precisely sure what a michtam (or miktam) was, and that’s why the Hebrew word remains as a transliteration in our English Bibles. Translators didn’t know how to translate michtam, so they spelled it phonetically and called it good enough.
Psalm 16 is titled “A miktam of David.” The other psalms that are called “michtams” are Psalms 56–60. All six of these are psalms of David. In Isaiah 38:9, King Hezekiah’s song is introduced with these words: “A writing of Hezekiah king of Judah after his illness and recovery.” The Hebrew word for “writing” here is miktab, which many scholars believe is related to michtam.
A possibly related word to michtam is the Hebrew katham, which means “an engraving.” If the underlying meaning of michtam is “engraving,” then the songs labeled as “michtams” could have been considered of enough value to be stamped or engraved upon tablets for long-term preservation. Some scholars see the word michtam as meaning “golden,” a definition that would similarly assign great value to a song so labeled. A michtam could be “a psalm as precious as stamped gold”; if so, today’s top-selling songs that are “certified gold” could be considered “michtams” of a sort.
The link between a michtam and golden worth is speculative, however. Other scholars think the word michtam is simply a technical term to guide the singer or to denote the tune to be played. In the end, we don’t know. Like the words maskil, selah, and shigionoth, michtam remains somewhat of a mystery in the Hebrew song book.