Why is this so hard to understand youngsters?!
Back in the day, there were the radio hits. Whether they were bonafide hits or album cuts played on the radio. those other songs on the albums were known as "Deep Cuts".
Well, I think I qualify as an old fart here, and that's not really how I define it. I don't think it's as clear cut as songs that weren't singles. I think it has more to do with songs that wouldn't be considered as singles. Songs that don't fit the prevailing style of the album or that are too long for radio/MTV play. I generally only think a song or two from an album qualify, and they're generally towards the end. Since Maiden is cited as an example, To Tame a Land is a fine example, as would be TLotLDR. Could this be Magic and Words in a Simple Rhyme are not singles material.
A definition I see that would jibe with the description is a song that's not as accessible than the others.
Also, since albums aren't really a thing anymore, I'd say that the term really has no current use. Anybody can listen to any song they like now.