Yellow battery

This means you are currently on Low-Power Mode (I'll link a guide below so you can understand more about what this means). Simply, it is prompted to be turned on when your phone reaches 20% charge in an effort to save your battery life, however it can be enabled at any time. To disable this go to Settings > Battery > Low Power Mode.

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

That means that your iPhone is set to Low Power Mode.

To turn OFF Low Power Mode, Open Settings 👉 Battery ➡️ Low Power Mode: Set this switch to OFF.

Axel F.

That means it is in low power mode.

You are welcome.

Axel F.