After replacing the battery on your iPhone, the message "Important Message" will appear on the screen.

• The message only informs that the original battery has been replaced.
• The message has no effect on the performance of the device.
• The message is displayed on the lock screen for 4 days, and is then visible in settings only

This is due to the fact that the replacement was not approved by Apple's official software, and the part was not paired with the motherboard. The battery can be programmed with the correct tools and knowledge to bypass this message.