PostEaseThe short answer
A content mix is the deliberate balance of different content types an account publishes over time (educational, promotional, entertaining, personal), usually expressed as a rough proportion like the 80/20 rule. It is a planning tool for making sure a calendar does not skew entirely toward one type of content.
Why the mix tends to drift without deliberate attention
Left unplanned, a content mix drifts toward whichever type is easiest to produce in a given week, often promotional, since it requires the least original thought. Planning the mix deliberately counteracts that drift.
A reasonable way to track it
Tagging each planned post by type in a content calendar, and periodically checking the actual proportion against the intended mix, is a simple way to catch drift before a month goes by leaning far more promotional (or far more of any single type) than intended.