added definition of what pre-rendering is

Author: chris-nowicki

docs/storefront/stencil/start/index.mdx

@@ -56,6 +56,8 @@ Stencil themes can access remote objects through event hook and use the hooks to
 
 All stencil themes utilize the [Speculation Rules API](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API) to pre-render checkout pages, improving performance and user experience.
 
+**Pre-rendering** means the browser begins loading a page in the background (for example, when a shopper hovers over or is likely to click a checkout link) so that it appears almost instantly when opened.  
+
 ### Limitations
 
 - Currently only works if the shopper is using the Google Chrome browser.  Check [browser compatibility](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API#browser_compatibility) to see latest browser support.