A SIM-only plan is a mobile phone service that provides only the SIM card without including a new phone. This type of plan allows users to keep their existing device while enjoying a monthly contract that typically offers data, calls, and text services. These plans are often more affordable than traditional contracts that bundle a phone with service. Users can choose from various options based on their usage needs, making SIM-only plans a flexible choice for those who want to save money or avoid long-term commitments.