Usually in reference to buying links on Wordpress Themes. The footer is encrypted, so that when people download it, they have difficulty removing the "sponsored links" before using the theme on their blogs. Almost all encrypted footers can be easily bypassed by simply installing the theme, right click page, then "view source".
Yeah, just copy/paste the footer section of html from "view source" into your theme's footer.php file, change it, upload it, and you should be good to go. The only time this doesn't work is if the theme has some essential php coding in the footer that is required for the theme to run properly, but the large majority of themes don't do this.
About 90% of the time, this will work. Don't take my word for it. Just test it out and see. If it doesn't work, then you got a theme that falls in that 10% that doesn't work with this process. Either get a different theme or find another way to decode it.
Exactly as live-cms says. The encrypted footer is decrypted on the server side. It then spits out the HTML for your browser to read. When you right click the page and click "view source", you will be able to see the raw HTML source code after it has been decrypted. You just copy/paste this footer section of the source code into your footer and you're good to go.