"For Official Use Only." With respect to information in a US Government context this implies that the information is not classified (e.g., secret, top secret, etc.), but it is not public, either. The information can be shared with anyone with a clear need to know, but the information should not be publicized. In other contexts (e.g., at a specific company, or with respect to the usage of a government vehicle), it means something slightly different, but the spirit is the same.
Anything that has to do with High Importance
For Official Use Only (FOUO) is a way to classify documents, in particular, documents produced by the Department of Defense. FOUO can include all documentation that is deemed not appropriate for the public even if it is not classified. This does not mean that the documents cannot be requested through the proper channels; they can under FOIA laws.