The Wikipedia gives “the elephant in the room”” the following explanation:
The expression “the elephant in the room” is a metaphorical idiom in English for an important or enormous topic, question, or controversial issue that is obvious or that everyone knows about but no one mentions or wants to discuss because it makes at least some of them uncomfortable and is personally, socially, or politically embarrassing, controversial, inflammatory, or dangerous.
In his first speech before Congress, President Biden proposed the largest job plan, higher taxes on wealthy individual and corporation, raising the minimum wage, cutting the price on prescription drugs, immigration reform, gun control and racism. These are social and political issues most people face and most politician avoid to talk about in decades. So, Joe Biden’s speech is “the elephant in the room”.