There is a myriad of macros that can be used in the Confluence Server/DC that are not compatible with the Cloud environment.
When you migrate a space that contains pages with macros that are not compatible with Cloud the macro breaks and start showing a warning box with "unknown macro":
The incompatible macros code would be stripped during import, leaving the text part inside the macro intact.
A warning box is shown saying the macro is unknown.
You can remove the macros on the server prior to migrating to the Cloud.