You can read all about the probable origins of this quote in Quote Investigator
The way I understand this quote is this: minor artists try too hard to imitate that their work ends up being a caricature or facsimile of the original. Whereas great artists have so much .... that they "steal" the essence of what was their inspiration; it is now their own.
Let's face it, if you think your photographic ideas are totally original, chances are you are hugely mistaken. Nothing is born in a vacuum; even insanely innovative photographers draw their inspiration from the work and style of others before them.
Still, it sometimes it may be annoying to discover that someone is imitating your style in a particular shooting scenario, or even downright "copying" your frames. This is something I've came across a couple of times while shooting live music performances: a guy (usually a low-paid photography student, assigned by one of those "free promotion" agencies) will follow me around, taking the exact same photo (or trying to, since I have no way to review his shots), copying the exact angle I'd use.
A couple of days later I'd see their photos uploaded on Facebook and such, and, sure enough, they will be nothing like mine, just clumsy attempts at best. Still, the effort counts, and I say this with no hint of sarcasm. I'm a strong believer in "fake it 'till you make it". I also believe in "imitation is the sincerest form of flattery". So, yes, I don't mind at all.
Not only that, but sometimes I really enjoy it: a while back I was shooting a live band on assignment, when I noticed the son of the bass player (and friend) "copying my frames" with his smartphone. And doing a bloody good job at that. And I'm pretty sure, he will soon be in the "great artists" category on the title.
Because, a little later, the band invited him on-stage, and he proceeded to blow us away with his drum playing.
Way to go kid! Keep it simple; and steal like hell!