I have a large collection of photos (80Gb) stored on my Macbook and I've imported them to Photos, these are now available through Photos on my other devices with the same iCloud account. Does Apple move the originals into the cloud and free up the 80 Gb space on my Macbook?
No - You don't initially save any space on your Mac if you choose iCloud storage.
Yes - if you set "optimize" in Photos preferences, then once you fill your storage with other files, the Photos library will shrink itself.
You can choose to store the original size images in the cloud and let the Photos app "optimize" and resample/recompress images to reduce the storage space burden on selected devices.
For example, if your 80 GB library is half RAW files and/or very large JPEG files, you might save a substantial amount of space by selecting Optimize and then filling up your storage with other files to trigger the optimization.
As you can see from Photo preferences, only when you run low on storage space will the optimization start to take place. I've not tested this yet, but hope to try it on a second Mac with limited storage to see how the library looks when space runs low and "optimization" kicks in.