What the “F”!

I’ve been having an issue with GoDaddy about redirects. It involves things I’d like to post about, but they’re still being identified. But in the process the subject of .htaccess files came up, and something I noticed from one file to another was the addition of the letter “F” to these bits of code.
Could someone explain to me what this letter means and what it does?

RewriteRule ^wp-admin/includes/ – [F,L]
RewriteRule !^wp-includes/ – [S=3] RewriteRule ^wp-includes/[^/]+\.php$ – [F,L]
RewriteRule ^wp-includes/js/tinymce/langs/.+\.php – [F,L]
RewriteRule ^wp-includes/theme-compat/ – [F,L]