SEO is the topic and duplicate content is the issue. When there is duplicate content, canonical tags tell search engines which page to index. They ensure that you won't be punished for having content appear multiple times on your site. They also allow search engines to link to the dominant page out of the bunch.
To add the tags:
Pick the page that you want to be indexed, and write down that URL.
Go to the duplicate pages that should not be indexed, and insert this code in the <head> section: <link rel="canonical" href="INSERT INDEXED PAGE URL"/>
If you have any questions, follow these instructions from Google.
- If you enjoy more technical explanations, you will find a thorough review of canonical tags on MOZ here.
Canonical Tags can also apply to content that is syndicated when publishing across many domains. Matt Cutts from Google talks about it in the video below:
Another very quick video from Matt Cutts about when to use canonical tags:
Questions? Contact me here.