ReadyBoost - You May Not Know About This

ReadyBoost enables NAND memory mass storage devices, including CompactFlash, SD cards, and USB flash drives, to be used as a write cache between a hard drive and random access memory in an effort to increase computing performance.​

So in a nutshell, this is how you do it and what it's for:

If you have a low-spec or slower machine you need a performance boost for, you can insert a thumbdrive into a USB slot, and it'll act as a cache for your most commonly done tasks.  

Once you insert the thumbdrive, right click it in "Computer/This Pc," and select properties.  From there, you will see a ReadyBoost Tab.

Select "Use This Device."  I always use the whole drive because thumbdrives are very cheap nowadays, and the more space you allow it to use, the more you will see a "boost" in your computer-related activities. 

Hopefully you will see an improvement in your older low-spec machines as I have (although how much improvement will vary).

Note: If you try and use this with a high-spec machine, it'll tell you ReadyBoost isn't needed.