Secondary (indirect) immunofluorescence (SIF) is similar to direct immunofluorescence, however the technique utilizes two types of antibodies whereas only one of them have a conjugated fluorophore. The antibody with the conjugated fluorophore is referred to as the secondary antibody, while the unconjugated is referred to as the primary antibody.
The principle of this technique is that the primary antibody specifically binds to the epitope on the target molecule, whereas the secondary antibody, with the conjugated fluorophore, recognizes and binds to the primary antibody.  
This technique is considered to be more sensitive than primary immunofluorescence, because multiple secondary antibodies can bind to the same primary antibody. The increased number of fluorophore molecules per antigen increases the amount of emitted light, and thus amplifies the signal. There are different methods for attaining a higher fluorophore-antigen ratio such as the Avidin-Biotin Complex (ABC method) and Labeled Streptavidin-Biotin (LSAB method).