There are two options for saving your project. Save and export. But I have no idea what different they are. Their function seems exactly the same. Isn’t it?
In case of export, the filename does not change. So, if you are working on “Funny-Picture.kra” and you export your work as “Big-Mountain.png”, your project will be “Funny-Picture.kra”, in case of save to “Big-Mountain.png”, your project will be “Big-Mountain.png”.
Michelist
Great! Much appreciate it 
Hmm…kinda complicated. I’ll just leave it alone. It’s not a big deal anyway. I was just curious.
Thank you!
Typically, you want to use “Export” when you want to save in a different format than .kra, for example you finished the work and you want to show your friends or publush it, then you Export as png. That way Krita will still remind you to save as .kra.
I nearly never use Save As, only Save Incremental Version to save different stages of my project. Works the same way, except in case of Save As you provide the filename yourself.
The main difference is that Save as… will make the saved copy the new active document you work on, while export does not.
This can easily make you lose all layers when saving as a png for example and you didn’t notice your not on your original kra file anymore.