To what is göring referring when he says "the German luftwaffe has struck at the heart of the enemy"

This was part of Goring's speech when the Luftwaffe first started dropping bombs on London. Ironically it all started with an error which turned into a history turning moment.

A lost Luffwaffe bomber accidentally dropped its bombs on London. This had been prohibited by Hitler. The Brits responded by dropping bombs on Berlin. This infuriated Hitler so much he ordered London be bombed. As the German bombers flew toward London, Goring gave his speech which included the phrase you quoted.

Ironically the bombing of London, which was terrible for the city, gave the RAF airfields a break from being hit repeatedly so they regained strength to keep on fighting the Germans.