Lots of messages…are they errors? how to correct??

Hi there,
I’ve set up my W3 Cache according to your instructions; but I have a few additional questions:

1. My site is still very slow. I’ve deleted plug ins, and done everything I could. Every time I change a post/page or delete plug ins, I am getting the following message that pops up all the time on the W3 Total Cache main settings page:

One or more plugins have been activated or deactivated, please [empty the page cache] and check your [minify settings] to maintain the desired user experience.

2. When I “check minify settings” what does THAT mean?? check what???

3. Here is another message I keep getting:
The setting change(s) made either invalidate your cached data or modify the behavior of your site. now to provide a consistent user experience. Does this mean that W3 Total Cache is not working? Invalidated how??

3. There is also a message at the bottom of the settings pages that says:

* Enable HTTP compression in the “Cascading Style Sheets & JavaScript” section on Browser Cache Settings tab. When I check these settings, I see that the HTTP compression IS set

* The TTL of page cache files is set via the “Expires header lifetime” field in the “Cascading Style Sheets & JavaScript” section on Browser Cache Settings tab. What does this mean??? Should I set the expires header lifetime?? HOW do I set it???