High-Level Summary:
There was a ton of short-selling interest on TSLA (due to expectations of a big earnings miss). Almost 27% of the 115mm shares outstanding are currently being borrowed by short sellers. TSLA has more short interest by percentage than 98% of US stocks. Everyone was really expecting the price to go down.
Tesla reported earnings today at $.12/share, and upped their forward guidance. The consensus earnings estimates were $.04/share, so TSLA greatly surpassed expectations.
To short a stock, you have to borrow a share from someone else, and then return that stock to them at a later date. Returning the stock is called 'Covering a short'. All those people who were betting against TSLA are now forced to pile back into the market to cover, but since so many shares were short to begin with, the number of people who have stock to sell is much lower than typical.
This results in a 'short covering rally' where there is a lot of demand to buy shares and a small supply. Econ. 101 takes over and you see a big spike in the price.