Satoshi, made this statement in an email to Mike Hearn on October 28, 2010. In this context, Satoshi is referring to the automatic adjustment of the Bitcoin network's difficulty level.

The "difficulty" in Bitcoin refers to the computational power required to solve the complex mathematical problems needed to mine new blocks and validate transactions. As more miners join the network, the difficulty level increases, making it harder to solve these problems.

In this specific case, Satoshi mentions that the difficulty has increased by a factor of 4, meaning it is now four times harder to mine new Bitcoins. This increase in difficulty has a direct impact on the cost of mining.

Satoshi then mentions that the cost of mining is now US$0.02 per Bitcoin (BTC). This means that, at the time, it cost 2 cents to mine one Bitcoin. The cost includes the electricity and computational resources required to solve the complex mathematical problems.

In essence, Satoshi is highlighting the dynamic nature of the Bitcoin network, where the difficulty level adjusts to the number of miners and the computational power they contribute. This adjustment ensures the security and integrity of the network, as well as the decentralized and competitive nature of the mining process.