What is Cross-linking? What is the function of Crosslinking?

This is when multiple sites that you own link to each other. However, they will now quickly get you a Google penalty, ever since G changed their algorithm to focus on PBNs. (Public Blog Networks, or PBNs, are when you have a private network of lots of content sites all linking to each other. Typically sites in a PBN are rebuilt from expired domains that might already have incoming links, etc.)

https://searchengineland.com/private-blog-networks-great-way-get-site-penalized-286489

Cross linking is used to refer the process of linking one site to another site and provide a way to allow the accessing to it.