beginner question: how does shared storage work

Hello,

I am sorry if this question is already covered, I did not find it somewhere that was understandable to me.

I have a proxmox host, and on this host I have a local directory added as storage (/data/). Now I want to access this storage from inside VMs. How do I do this?

Also I should mention that there is already data in this directory that I would hate to loose.

Best regards
Daniel