Mounting just means that the print is professionally affixed to a single-weight mount board so that it doesn’t warp or wrinkle with time and changing climates (which is nearly guaranteed to happen here in Hawaii to the larger prints that are not mounted). I won’t even sell them unmounted here because they warp with the humidity. My favorite photo of my son when he was born warped and got all wavy under the glass (I chose not to mount it — stupidly).
And for those of us that move every couple of years or so, I also recommend that everything be matted when you frame it because of our lifestyle and changing climates that we move in and out of. High humidity areas can cause a print to stick to the glass without a mat. I’ve had some old family photos of grandparents that this has happened to and I’ve had to pay to have them restored.
For any print that you want to have on your walls for years to come, I would have it professionally framed, especially if it is something that you’d want to keep on your walls forever. The professional framing seals the photo and helps it from letting in moisture, dust, bugs, etc. I’ve learned my lessons (the hard way) and now I get anything that I think I want to be hanging on my walls for years to come professionally printed and framed and just frame the rest myself with frames I buy at various stores. Yes, even I buy some frames at Target. 😉