Synthetic mode is when something is made to look or feel like something else. It's like when you play dress up and put on a costume to pretend to be a superhero or a princess.
In technology, synthetic mode is used to make one thing act like another thing. Imagine you have a computer program that is supposed to talk to a printer. But the printer you have is not compatible with the program. So to solve this problem, you can create a "synthetic mode" that makes the printer act like it's a different kind of printer that the program can talk to.
It's kind of like a magician's trick, making something appear to be something it's not. But in the case of synthetic mode, it's done with technology to make things work together even if they're not completely compatible.