Biology-culture fit is the extent to which cultural adaptations fit onto our biological anchors. For example, we can't make biological ants believe in a Sun God. Ants' simple biology isn't a good anchor a complex culture.
For another example, let's look at how our cultural marriage norms were built on our biological pair-bonding instincts. As homo sapiens moved away from apes, we developed pair-bonding instincts to have a male-female pair watch over their children as their brains developed. From that biological instinct (pair-bonding), we developed the cultural technology to create marriage norms like wedding vows. But if we didn't have our pair-bonding anchor, those marriage norms wouldn't have stuck. Imagine trying to get sex-happy polygynous bonobos to settle down with one partner for life!
We can only jump so much in our evolution at any given time. This leads to a certain kind of "path dependence". Homo sapiens in 2020 are who we are because of homo sapiens in 10,000 BCE. And homo sapiens in 3000 can only follow from homo sapiens in 2020.
In technology, we'll often call this the "law of the adjacent possible." The iPhone wasn't possible in 1930s before computers. We needed information theory, transistors, digital cameras, and GPS before we could build the iPhone.
As another technological metaphor, think of Product-Market Fit (PMFit). PMFit says that a product will only be successful if it meets the needs of the market. Similarly, a given cultural innovation is only possible if it fits our natural biological anchor. We could call this Biological-Cultural Fit, or BCFit.