site maps and refresh date confusion

Can anyone explain site maps to me?

[Pro_Editor]

Sorry for being stupid, but I maintain my own biz site and could use an explanation of site maps. I implemented my G-freindly xml map last time my site dropped severely, but it does not seem to do what I thought it was supposed to do.

I assumed the map was dynamic and the 'last modified' date would change each time I worked on a page, but that seems really naive now (since the date and time of implementation are displayed no matter what). Do I simply change the date in the site map file?

I also thought a map triggered G to visit pages on the basis of the update period (daily, weekly, monthly, etc.), but when I do a site: search, the refresh dates for the pages are all over the place, in spite of most pages saying "weekly." All page links are available from the main page, which was crawled yesterday, but no other pages have that date.

Any explanation would be greatly appreciated.