What does yellow battery status bar on my iPhone indicate?

You have Low Power Mode turned on.

Use Low Power Mode - Apple Support

When Low Power Mode is on, the battery in the status bar will be yellow. You'll see a yellow battery icon and the battery percentage. After you charge your iPhone or iPad to 80% or higher, Low Power Mode automatically turns off.

It means you turned on Low Power Mode. To turn it off, open the Settings App. Find and tap Battery. Turn off Low Power Mode.