You will have read how important it is to create backlinks to your site and of course that is a key aspect of improving your search engine ranking. Trackbacks are an excellent means for building such backlinks off other Blogs.
It is very well explained in an article entitled "How Trackbacks Work".
This technique is applicable to Blog articles you read on someone else's website which you would either like to comment on or write your own Blog post on that subject. Now, if you were to leave a comment on their Blog fine you will have an entry but not necessarily a link back to your web site. What you can do with Trackbacks is essentially place a post on your own site with a reference to the original Blog post and with Trackbacks you establish a link between their post and yours.
On their site you will see your post listed as a comment as extracted from your own Blog and within your Blog post their will be a link to their Blog post.
Note that this is not the same as placing a hyperlink and you must follow the instructions given in the above article. Another thing you will need to identify is that the article is in a blog that supports Trackbacks. Not all posts will have this facility set or available.
Trackbacks are a special link established between two addresses (the location of the Blog posts) that "pings" each other to ensure that the link remains live. If you find a Blog post that has a good Google ranking then even better.