How to I add code to my public file? What is a public file?

I have gone through my multisite steps up through the point of choosing sub domains. I now have a page, "Enabling the Network" where it has two sets of code for me to copy and into a public file.

The first one is to be pasted above the line "That's all, stop editing" etc. (I think I have already done this)

The line says, "Add the following to your .htaccess file in /home/.../public_html/, replacing other WordPress rules:
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L

Where am I supposed to do this? What is my public/html file? Where do I find it?