Question "Cannot use battery and power charge at the same time"?

May 23, 2019
Hi. I have a HP 1000-1202TU laptop. A few days ago, it couldn't turn on, so I brought it to a laptop maintenance shop to get it fixed.
After looking and checking some components of my laptop, they said that there was a problem with the motherboard. After a while, they fixed it and my laptop could turn on, but they told me that from now on, I couldn't use battery and power charge at the same time. They said I could use either the battery or the charger, it's ok to do that, but not both of them.
Since I don't know much about hardware stuff, I just listened to their advice and did what they told. But I'm still curious about that. So I'd like to know if anyone here has faced a problem like mine before, and if you has, please explain it with more details to me. Thank you.
Ps: I'm a total hardware noob, so I think my question may lack technical information. Sorry about that.