PageSpeed Insights in a tool offered by Google to help you optimize your page speed and give you suggestions about how your site can do better. Below we cover a few of the common recommendations from PageSpeed Insights.
Leverage Browser Caching
To fix the "Leverage Browser Caching" warning given by PageSpeed Insights you need to make sure that your are returning a Cache-Control header. For more information on that see
Cache Control Header.
Optimize CSS Delivery
To resolve the "Optimize CSS Delivery" warning from PageSpeed Insights have a look at
Cache Control Header. This is an advanced topic so proceed with caution.
Client Resource Manager.