What does it mean when the battery icon turns yellow after you charge your phone?

I may have or is it possible that my device can go into that mode automatically as indication of anything specific? over use and/or over charging in short period of time?

do you recommend changing it back?

Will it affect anything negatively or can I keep it as is and will it switch back auto? Curious

Thanks!

kbf-iCloud

It is an indication that you have turned on the Low Power Mode option.

It will have no negative impact on the battery.

Use Low Power Mode to save battery life on your iPhone - Apple Support

You’re quite welcome.