What does Google consider an Update?

Phil

I've been wondering about site updating and Google. I've always heard that Google likes a site to be updated regularly.
Well, my site has over 150 pages. And I find I'll update the sub-pages quite often, but the main, index page may not get updated as much. I'm always adding pics and little bits of info or changing something.
So, I guess my question is "Just what does Google consider updating?" The main page or the content contained in the site? And how much is enough for Google to even recognize it as an update?

the-guide

Normally, the homepage always shown that what's change and what's updated inside your entirely site.

I think Google consider the change and update on both homepage and contents inside entirely site, homepage might be more important for Google but the contents inside your entirely site also important too.