Like their names said:
“Memory Cache” stores and loads resources to and from Memory (RAM). So this is much faster but it is non-persistent. Content is available until you close the Browser.
“Disk Cache” is persistent. Cached resources are stored and loaded to and from disk.
Simple Test:
Open Chrome Developper Tools / Network. Reload a page multiple times. The table column “Size” will tell you that some files are loaded “from memory cache”.
Now close the browser, open Developper Tools / Network again and load that page again. All cached files are loaded “from disk cache” now, because your memory cache is empty.