What does the yellow charging mean?

It means that you have enabled Low Power Mode on your phone.

Settings > Battery > Low Power Mode

Move the slider to the left to turn off Low Power Mode.

According to Use Low Power Mode to save battery life on your iPhone or iPad, "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."