What does the site: operator do?
The site: operator allows you to see how many pages of your website are in the index of a major search engine.

How do I use the site: operator?
It's really easy to use the site: operator in any major search engine. All you need to do is type site: followed by your website address. Here's an example for SEOhub:-

Example: site:www.seohub.com.au

If I were to type this into a search engine I would be able to see how many pages are in the index.

How do I tell how many pages are in the index?
After you've gone to your search engine of choice and added your site: operator and your website into the search box and hit go, you should see a results page that will give you the amount of pages in the index. It should look something like this:-

Results 1 - 10 of about 145 from www.seohub.com.au

My pages aren't in the index, what should I do?
The first thing you will want to do is make sure your website is crawlable. You can use our spider simulator to see if your website is crawlable. Alternatively you may wish to read How to make your website crawlable by search engines.

When I get to the last page I see a message that says:-

In order to show you the most relevant results, we have omitted some entries very similar to the 115 already displayed.
If you like, you can repeat the search with the omitted results included.

What does this mean?
This means that you have pages which Google deem to be duplicate content. This may have implications on your rankings and you will want to ensure that these pages aren't hurting your website. Check out What is duplicate content? for more information on what duplicate content is.