The caching mechanism implemented in Joomla is a way to improve the performance and loading speed of the content in your Joomla website. When the caching is enabled, Joomla creates a copy of the content of its database and stores it on the hard disk of the server. When someone accesses your website with request to see certain parts of your site, Joomla will check the cache for the searched content and will display it much quicker than if it had to send the request as query to the MySql database.
You can turn the caching on via your Joomla admin area -> Site -> Global Configuration -> Cache Settings -> YES.
You can also edit the "Cache time" from the same menu. Note that the time is measured in seconds and it represents the the time for which the cache file will be stored before it is refreshed. If you update your website constantly, then it is a good idea to select more frequent refresh rate, however if the website is oriented more towards static content, then you can set a refresh rate of 12 or 24 hours. For example if you want to set 12 hours cache time, you should set 43200(seconds).
In most of the cases the folder where the cache is stored is called /cache and is located in the root folder of your script. This directory must be writable for caching to be successfully implemented.