Next to the page name is a little plus icon. Click on it and you'll get all the searches that ever led to this page. Each of those searches will have numbers of its own, or a cross-link to the Top Searches for that term. In general they cut off at ten impressions or ten clicks. (This is an annoyance when you have huge numbers of searches for terms you're not even remotely interested in.)
I've always assumed it was a straight arithmetic mean: multiply each individual search by its ranking, add them up, divide.
The only page that has the plus icon is my home page. I guess because the other pages have <10 clicks means no + icon. Is there some other technique or online software I can use to see what keywords are making the pages occasionally show up in Googles's search results?
If you take the other tab, Top Searches, it always tells you what page(s) the search leads to. In fact you can often tell just from the search term, like if you've only got one page that talks about foo and bar then you know where the "foo + bar" search will lead you. I generally find this tab more useful, except when there's a page that garners a whole lot of weird, one-off searches so no single one shows up on the "top searches" list. But you can get this information from your logs too.
:: pondering the weird fact that searches for "warble fly"* previously led only to the non-www version of a page (I've only recently consolidated them in both g### and onsite) ::
* The larvae, to be exact. You don't want to know the details. Trust me ;)
Again thanks for your help and please allow me to apologize in advance for my next question but I don't see under the "top queries" tab where it lists the page that the search led to. How do I get that information?