I am completely new to Raspberry Pi, I have done a pi course though, but to be safe assume I know nothing about Pis. I have a working emulator that uses a Raspberry Pi 3B, I bought a Pi 3B+ and a 5V power supply. I got the lightning bolt in the corner of my screen when I was using the power supply I bought, I tried using the one that was successfully powering the Pi B, but when I tried it on the new Pi B+ it also got lightning bolt.
I believe they both take the same power supply. I searched online and it told me that the lightning bolt means the power supply is bad.