ldd is a unix command-line tool to print shared library dependencies for an executable or shared library.
If a program is dynamically linked, i.e. uses shared libraries, the program requires some number of external libraries in order to run. ldd is a tool to list the required libraries and shows if those libraries can be found in the current configuration.
Questions about using and understanding ldd output should use this tag.
From a unix command-line running
man ldd
will provide more information.