For example, you have installed web2py in mydomain.com root web folder, and written your application called "myapp". By default you can access your application through mydomain.com/myapp/default/index. However, sometimes, it is needed to have it in the way, like mydomain.com/default/index, and all other installed applications (f.e. admin) should be still accessible in just normal way as mydomain.com/admin/default/index.
This task can be easily solved with help of mod_rewrite. Just check if your provider support it (normally it is the case). And put the next file in your root web folder.
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !^/myapp
RewriteCond %{REQUEST_URI} !^/admin
RewriteCond %{REQUEST_URI} !^/some_other_exception
RewriteRule ^(.*) /myapp/$1 [L]
and save this file as ".htaccess"
With this file, every URL that does not start with /admin/ or /some_other_exception/ will be routed to /myapp/. For example, if user calls mydomain.com/some_cool_stuff/index it will be redirected to mydomain/myapp/some_cool_stuff/index. The redirection will be done internally, so user doesn't see "my_app" in navigation panel of browser.