The title is a phrase you see and hear a lot when talking about games - and also stories. It is an interesting idea. You often see it in response to "you got X idea from Y" source, and it's not untrue. I fully endorse borrowing and theft for games your running, stories your telling, and almost every other creative endeavor. However, what does the phrase actually mean? How do you borrow something? How do you steal something? That I find is a bit unclear. Let's unpack it a little bit.
When you borrow something you are taking something that is someone else's and using it. This comes with certain caveats. First of all, the thing is not yours. This means you don't break it. You don't change it. You just use it as it is.
For example, if I'm running an old 7th Sea game where it was the original version of Roll and Keep, I could borrow the mass battle tables from L5R to run mass combat. Yeah, I can't necessarily have my pirate fight a Shadowlands creature (or can I?) but I can just use the table.
However, this phrase is often used for story beats more than design elements. So what would a story element be? The Chroma Conclave from Critical Role Season 1 is a huge story. It's a group of ancient chromatic dragons that make a bid to take over the game world and are successful at it too for a time. You could easily lift that idea, have a mixed group of chromatic dragons, and put it in your game.
If you have players that enjoy Critical Role some of them will notice the connection. You don't have to shy from it. Your game will be your game and thus play out differently. You've borrowed the idea from Matt Mercer - and it is a good idea - but it is still your game with your players.
There is no shame in this. Remember, good GMs borrow. You see something you like, borrow it.
The act of stealing is taking something that is not yours and making it yours. See the difference? Yeah, stealing has other meanings, but the big difference for our purposes between borrowing and stealing is ownership.
Ownership when it comes to ideas means changing it. It's not enough to take Matt Mercer's Chroma Conclave, you need to make the idea your own. Yeah, the inspiration will always be there. Yeah, the connection will always be there. But in making it yours you also leave room for doubt. Mercer isn't the only one who has ever though of multiple dragon types working together.
Now, how you make that idea yours is something I can't tell you. I don't know you. I don't know your table. But it means changing it, twisting it, shifting it, and shaping it so you own it. It means understanding it and knowing how to use it.
This can be done multiple ways. Maybe instead of Dragons it is 5 powerful wizards, or devils, or something else. Maybe the motivations are different. Maybe something else is different. Maybe they're not antagonists but support for the heroes. Maybe they're just their own thing and it is up to the PCs to decide if they're enemies, friends, or something in between.
Good GMs Borrow. They take ideas they like from other places - even their players - and implement them into their games and worlds to make the thing better. Great GMs steal. They take those ideas, and they twist them and work them to add their own bit to them. You have within yourself boundless worlds and stories. Trust me, no matter who you are, you have something to add to any idea to make it better for your game and your group.