Custom firmware is modified software that replaces the original firmware on devices like routers, gaming consoles, or smartphones. It allows users to unlock additional features, improve performance, or enhance security beyond what the manufacturer provides. Users often install custom firmware to gain more control over their devices, enabling them to customize settings and functionalities. Popular examples include DD-WRT for routers and LineageOS for smartphones. However, using custom firmware can void warranties and may lead to potential risks if not done correctly.