Final answer:
A compass is the tool that indicates direction by aligning with Earth's magnetic field, essential for navigation.
Step-by-step explanation:
The tool that helps a user know what direction they are headed is a compass. A compass has a magnetized needle that aligns with the Earth's magnetic field, which allows the user to determine the cardinal directions such as north, east, south, and west. This is an essential tool for navigation and is widely used in different fields, including outdoor activities, military operations, and surveying. While other instruments like a ruler, telescope, and protractor also serve important functions, it is the compass that is specifically designed to indicate direction based on magnetic orientation.