"Lossless" means you're not making any permanent changes to the original file. For example, if you open a jpeg in Photoshop, change the brightness and save the file, that file is changed permanently.
In LR, if you work on a raw file and make a "change", what LR records in instructions about what to change. LR doesn't really change the file itself (this may be different for jpegs or tiffs, I don't know).
You can revert to the original by resetting the edits or by making a virtual copy and resetting the edits there. That way you can have you edits and the original.
Another way to do this is to make the edits you want and save a snapshot. then you can revert to the edited image or the original just to see what you've done.
I hope I'm clear -- it's very clear in my head. Let me know if I'm not.