From The New York Times,

As the dust settles from the presidential race, the eyes of the political world have already shifted to Georgia, where two runoff elections set for early January will almost certainly determine which party has control of the Senate.

The outcome of the contests, which will play out two weeks before President-elect Joseph R. Biden Jr.’s inauguration, will either swing the majority to Democrats, handing the new president broad power to carry out his policy agenda and push through nominations as he sees fit, or leave Republicans in charge, allowing them to influence his plans. [...]

Democrats would need to capture both of the seats in Georgia to secure a 50-50 tie in the Senate. Then, Vice President-elect Kamala Harris could cast tiebreaking votes to carry out the Democratic agenda. If they were to lose one, Republicans would maintain their majority, albeit by the slimmest of margins.