Joe Biden has a long-standing relationship with Burisma Holdings, going back to his time as Vice President. Burisma is a Ukraine-based oil and gas conglomerate. As VEEP, Biden was tasked to oversee US dealings with Ukraine.
Biden's son, Hunter, was receiving up to $200,000 per month as a "board member" of Burisma, beginning after Biden was out of office. Biden has threatened to withhold over $1 billion in loan guarantees to Ukraine unless they fired Viktor Shokin, the Ukrainian prosecutor investigating Burisma's relationship with Hunter Biden.
The Democrats are asking us to ignore all this and investigate Trump for pressuring the Ukraine government to expose it. This smacks of the pot calling the kettle black.