In brief
break-inside controls breaks within a generated box, including page or column breaks. avoid expresses a preference not to split content there.
Useful to know
break-inside controls breaks within a generated box, including page or column breaks. avoid expresses a preference not to split content there. [1]
It does not promise that an arbitrarily large block will always fit. For print, check a real page with long content; the available area's limits can require a break despite that preference.
Sources
For further detail and to check the information: