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Expires headers let the browser know whether to server a cached version of the page. This can simultaneously help to reduce server load and increase page load time by telling the browser that it doesn’t have to check for new versions of files for an extended period.
The code below will allow images to be cached for a year and text-based content to be cached for one month. For most fairly static websites (like this one) that doesn’t change very often aside for a new post now and then this should be more then enough.
If you are running a dynamic website be careful with any sensitive information which may be stored, for example JSON responses, user account pages and the like should be set-up to not cache.
The above can be copied into your .htaccess file without any major issues but, of course best checked before deploying to a live environment.