Okay kiddo, this is a little bit complicated but I'll try to explain it in a way you can understand.
A synthetic chord is like a pretend musical chord that is made by a computer instead of by a person playing an instrument. Let's say you are playing a song on the piano and you want it to sound like there are more instruments playing than just the piano. You can use a special machine, called a synthesizer, to make a bunch of different sounds that together can make one big pretend chord. The sounds are made by the computer and can be adjusted to sound however you want them to.
It's kind of like making a cake. You usually need a few different ingredients like flour, sugar, and eggs to make a cake. In the same way, a synthetic chord is made up of a few different sounds that are combined together to make a full chord. The nice thing about a synthetic chord is that you can make it sound exactly how you want it to, since the computer can make any sound you can imagine.
So when you hear a song that sounds like there are lots of instruments playing, some of those sounds might be a synthetic chord made by a computer. It's like magic, right?