I listened a phrase "plenty of" in a video. I usually use "a lot of" to mean "many/much". Is there a difference between "a lot of" and "plenty of"?
They are very similar, and in most cases they are interchangeable. But, "plenty of" something implies enough of it. For example
"I have a lot of pokemon, but I haven't collected them all yet."
"I have plenty of oranges. Please, eat one of them."
You could switch them around, and neither would be grammatically incorrect. But since the goal of playing Pokemon is to collect all of them "plenty" doesn't quite feel right. Plenty is best used when you have so many you don't want any more. You can have so many Pokemon you can't remember all of their names, but still want to get more of them to finish the game.
"Enough" doesn't automatically imply "a lot." One baked potato can be enough for your dinner, but since it is just one it isn't a lot of potatoes. "Plenty" is like "a lot" with some sense of "enough" at the same time.