I first encountered popiah a few years ago in Singapore.
It is a fresh spring roll, commonly containing things like bean sprouts, lettuce, egg, grated turnip, sometimes meat, prawns or crab, maybe peanuts, all wrapped in a thin pancake. The fillings vary between vendors as do the toppings and the exact sauce. The vendors may also sell a deep-fried version tasting similar to spring rolls.
The upper example came from the same market as the apam balik, and the rolls were liberally covered in a hoisin-style sauce and sprinkled with nuts. These were fresh and crunchy, and the generous helping of sweet sauce left us licking our fingers.
The example below came from the New World Park Food Court in George Town, Penang (more on that later) and contained the sauce within, with a topping of what we decided were teeny tiny shrimp. I found these a bit drier and more bland, and I did not favour the shrimp over the peanuts of the previous incarnation.
Note that I think the dipping sauce in the top right of the photo was connected to another dish. I really must start writing more things down rather than relying on my memory…