iCloud Drive is the service from Apple to sync files between apple devices. If you want to set up it, check this guide, follow the steps on On your Mac https://support.apple.com/en-gb/HT204025

After you've have configured iCloud Drive on your device, assuming you currently have Stacks file saved on a local folder, you just need to move your current Workplace folder from the local folder to a iCloud Drive folder following the Moving a workspace guide.

After the files have been synced, to access the workspace in the other pc/mac you just need to install Stacks and follow the guide to add a new workspace, Adding a new workspace locating the iCloud Drive folder you have just synced.