Basically you input a website or webpage and some keywords you find relevant and the tool output relevant data like:
Volume of monthly searches
Advertiser competition
Suggested bid
If the keywords you entered were found on the website/webpage
Also you can filter by website category, branded or unbranded keywords and where the keyword appears: in keyword / in page / both.
Of course you can do all sorts of usefull stuff to the data like sorting and exporting, things we’ve come used to getting from Google over the years.
One more usefull thing is that you can click on a small "Google Insights" logo near every keyword and you will see the Google Trends data for that particular keyword.