What does "all cash offer" really mean?

[Mel Sims]

Hello! I just started back studying/reading about real estate investing, and the term "all cash offer" has me a bit confused. 

When someone has an "all cash offer" for an investment property, I know it's not really "green cash," but is it in the sense that an investor is paying for a property in full from the proof of funds in their account? Or is "all cash" just another term or scenario of how it's being financed? If it is indeed the money being used from the investors proof of funds, where, normally, does that money come from if they're just starting out in investing? 

I hope this makes sense...

Thanks!