Cache Expiration for Dummies (me)

I do not understand what google wants me to do about cache expiration

[donok]

Hello.

So, I have a client that is suddenly panicked about site speed (a whole 'nother discussion, there). But, FireFox's google pagespeed tool is telling me something about "The following resources are missing a cache expiration."

Where do I set this "cache expiration" - is it something my host sets in apache? Or in the individual page headers? Or in the img tag?

Sorry... I have no idea where to start.

Thanks,
Don

[jdMorgan]

See Apache mod_expires documentation, or set the "Expires" headers using php if the site is dynamically-generated.

In addition, you may wish to investigate the HTTP Cache-Control and Etag headers and their values, as they tie in with the Expires header to affect client-side caching behaviour.

Jim