Hot linking is using another website's image without their permission. Let's say I created a blog post in Wordpress and added a picture to my post. Instead of downloading it to my computer, and then uploading it to my website, I insert the image from an image URL. When my page loads, it uses bandwidth from the other website instead of my website to load.
Quite often, the hosting company of the website will charge a website based on the amount of bandwidth the use, so if other websites use their photos and get traffic on a large scale over a long period of time, it could cost the website a lot of money. Website owners prevent other websites from using their images and costing them money by making changes to their htaccess file for their website and essentially blocking them from displaying their images when the copying website loads their page.
There are plenty of websites that have images that are creative commons licensed like Wikipedia and Flicker, and you can use the Creative Commons search feature to find images across various sources that are CC licensed: http://search.creativecommons.org/