They are emissions credits. Every company receives some amount of "CO2 emission credits" from the government. These allow you to emit a certain amount of carbon dioxide. If you don't emit all the CO2 that your credits allow, you can sell those credits to other companies that need more than the government gives them.
The idea is to put a total limit on the amount of emissions in the country, while letting the market figure out where it makes most sense economically to invest in emission reduction.
Tesla makes only EV cars and so it doesn't need all the credits a typical gasoline car company would receive. So they sell them.
Technically just a little bit different from fracking as used in the oil/gas industry, since it doesn't create new fractures in the rock, it only expands existing ones. However it carries basically the same risks with at most a difference in magnitude.
There's an interesting case in Switzerland where they tried to drill one over an historically active fault line, without first doing a seismic risk assessment.