Refers to the situation where you need memory data that’s not already in the cacheA place to park data coming from slow memory so that you can access it quickly. It's built with SRAM memory, but it's organized so that you can figure out which parts haven't been used in a while so that they can be replaced with new cached items. Also, if you make a change to cached data, that change has to make its way back into the original slow storage., so you first need to load it from slower memory into the cache.
Inside the Internet of Things - Bryon Moyer Blog