If you browse to your WordPress Folders such as wp-content, you may see something like this:
That means, others can easily know your installed themes and plugins, which is very dangerous and easily to hacked by hackers.
There are two ways to solve this problem. The first method is create an empty index.php file and put under the folder. The other method is edit your .htaccess file, add an entry “Options -Indexes” (withour quotes).
The second method is a more advance and secure method as you need to put index.php file in each folders if you use the first method.