What is returned by %{REQUEST_FILENAME} and %{THE_REQUEST}?

I was just checking over our .htaccess file and it dawned on me, I have very little knowledge of this. The code below uses both. It works I just want understand it.

#remove / at the end of URL
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.+)/$ /$1 [L,R=301]

#remove /index.php at the end of URL
RewriteCond %{THE_REQUEST} ^.*/index.php
RewriteRule ^(.*)index.php$ /$1 [R=301,L]

Cheers, Mark

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