Technical reports are in depth studies of a particular technology or process, typically contracted by a federal agency or corporation. These technical reports typically address a very specialized subject, but in great detail, so they can be quite valuable sources of information.
Some technical reports are indexed with journals and can be found in the 'Find Articles' tab, but there are specialized indexes of technical report information, typically of reports sponsored by the US Government.
The TRAIL project is an initiative sponsored by academic libraries around the country to digitize older technical reports that are otherwise very difficult to find. TRAIL focuses on technical reports published before 1976, which oftern are not included in indexes to the technical report literature.
For example, much of the seminal work in nuclear physics and health effects of radition was done in the 1930's-1950's, prior to widespread electronic access to material.