It means "to be honest," and it is often used in Facebook
statuses. Another definition is an "honest", yet all-too-flattering comment about a person that liked your status
(offering a tbh) in order to receive the comment.
These posts are usually not very honest and I think people would be much more honest if we still had the honesty box
. The honesty box was anonymous, private, and as a result, extremely HONEST! Just call it the TBH Box and it will regain its popularity.