Yellow battery indicator. What doe this mean?

It means you turned on your iPhone's power saving mode by accident.

Settings > Battery > Turn off "Low Power Mode"

Generally it only asks if you want to turn this mode on when your battery is getting low, but you may have inadvertently turned it on another way. It can be turned on even if the iPhone is at 100%

It is supposed to ask you when the power gets low, and you may have accidentally tapped "yes" to activate instead of cancel. I thought it automatically turned off when it is charging but I'm not sure. Haven't had enough experience with Lowe Power Mode.

Glad I could help!