Poly coated bullets are basically what many of us are calling "powder coating." It's a form of plastic coating. It can be applied in a few different ways,, and has become quite popular in the last 10 years.
Less expensive than jacketed,, and often not much more expense than cast.
Basically, a good alternative for many to using cast. They don't smoke like case bullets when fired because the smoke comes from the lubes used in cast. They shoot cleaner in guns. They often give a bit better velocities than using a cast bullet that's the same.
And depending upon the coating,, you can often slightly increase the velocity over cast bullets. Just do not load them to jacketed bullet levels.
I powder coat all my cast bullets now. Once I tried it,, I never went back to lubing them. And in USPSA competition,, I see a very large portion of competitors now using poly or coated bullets. Some buy them, and some do them themselves. I do my own.