If your website has some pages which have highly similar content then that may prove detrimental to your SEO efforts. How you may ask?
Take for example the following 3 URLs
As you may notice the page is the same in all 3 examples. Assume that the main function of the page is to display listing of products and the querystring parametres just change the order of listing of those products and Google indexes all 3 variations in its search engine.
With 3 variations of the same page indexed i.e. the main content of the pages being the same the said page will not rank high on Google as it may see that the page has nothing new to offer to the visitor. Even when things have changed now and Google says that its algorithm will display the correct page with respect to the search terms its better to play safe.
Google gives you the option of defining your canonical page. A canonical page is the preferred version of a set of pages with highly similar content.
If you take the above example of http://www.example.com you can define the first page as the canonical page. To define it as such, open the page and go to the <head> section. Please the following line in the <head>
<link rel="canonical" href="http://www.example.com/some_page.php?arg1=val1"/>
Now, Google will know which page to treat as the canonical page i.e. the page to consider in its search results.
To read more you can view the following page on Google regarding canonical
Hope the above helped.