To work with the Federal Government, you are required to have the appropriate background checks completed. This is to ensure you are who you say you are, and that there is no reason for the government to be concerned with your entry into their work space.
The level of security clearance that you will need for a specific position will depend on the type of information you have access to.
The most common level of clearance granted is Enhanced Reliability, referred to as Reliability. To have access to protected or sensitive material, you would need a higher level of security clearance, most commonly a Secret Clearance.