Your system is running in CPU Safe mode. This is when SMC believes there is a risk of overheating if allowed to run normally.
I would start off running the on board diagnostics to see if an error pops up. If it does tell us the full details. Here’s how to get to it [https://support.apple.com/en-us/102550|Use Apple Diagnostics to test your Mac]
To check your battery and charging logic install this gem of an App [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] take a snapshot of the Apps main window while the charger is plugged in and post it here so we can see it as well.