Obviously you mean the baseurl. If so:
base url: URL to your CodeIgniter root. Typically this will be your
  base URL, | WITH a trailing slash.
Root in codeigniter specifically means that the position where you can append your controller to your url.
For example, if the root is  localhost/ci_installation/index.php/, then to access the mycont controller you should go to localhost/ci_installation/index.php/mycont.
So, instead of writing such a long link you can (after loading "url" helper) , replace the term localhost/ci_installation/index.php/ by base_url() and this function will return the same string url.
NOTE: if you hadn't appended index.php/ to your base_url in your config.php, then if you use base_url(), it will return something like that localhost/ci_installation/mycont. And that will not work, because you have to access your controllers from index.php, instead of that you can place a .htaccess file to your codeigniter installation position. Cope that the below code to it:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /imguplod/index.php/$1 [L]
And it should work :)