RewriteEngine On

The RewriteEngine On line enables the Apache mod_rewrite module, which allows for URL rewriting and redirection.

RewriteCond %{SERVER_PORT} 80

The RewriteCond %{SERVER_PORT} 80 line specifies a condition for the following RewriteRule. It checks for the server port being set to 80, which is the default HTTP port.

RewriteRule ^(.*)$ https://www.example.com/$1 [R,L]

The RewriteRule line specifies the actual URL redirection. Here, any incoming URL that matches the pattern ^(.*)$ (which includes any character any number of times) will be redirected to https://www.example.com/$1, where $1 is the matched pattern from the original URL.

The [R,L] flags at the end of the RewriteRule specify that the redirection should be a permanent redirect (R=301) and that this should be the last rule applied (L, which stands for "last").