Essence Extractor is a block added by Tinkers' Construct that allows you to store your experience levels for later use.
By right-clicking on it, you can store your experience inside the Extractor.
If you left-click on it, you get an Essence Crystal that will give you your Experience back as soon as you eat it.
An odd thing about this block is that spending your experience via an enchanting table or anvil will cause the extractor to forget that you withdrew your stored experience. This can be unfairly exploited to produce an infinite supply of experience. It is unknown whether this is an intended feature. Strangely enough, this block was removed by later editions of Tinkerer's Construct but is still found in Hexxit.