The implied threat is that Burisma would be subjected to Biden-inflicted harm if it failed to pay for protection
Protection rackets are indistinguishable from extortion rackets.
The Bidens had plenty of US govt muscle to backup the implied threats.
The implied threat was that Ukraine would permit the prosecutor who was already investigating Burisma to proceed with his investigation, and perhaps cripple the company. Joe Biden saved the company by threatening the Ukrainian government with loss of $1 billion in loan guarantees unless that prosecutor was immediately fired. By getting the prosecutor fired, Hunter proved that his outrageous salary was a bargain for Burisma.
Joe Biden's quid-pro-quo had tacitly informed Ukraine that the United States wanted Burisma to be excluded from any corruption investigations. Trump's phone call to Zelensky informed Ukraine that the U.S. no longer backed Burisma, so the investigation could proceed.