Sen Joe Manchin is saying that he would back a more liberal Supreme Court pick.
Why is it important?
Manchin along with Arizona Senator Kristin Sinema are middle of the road Democrats who hold power in how they vote. With the Senate evenly split 50-50 (with the VP casting the deciding vote), how they go is likely how the vote goes.
The Senate is the chamber that votes on a nominated SC Justice. It is decided by a simple majority. Asssuming (not guaranteed but...) the GOP reject the Presidents nomination, and Manchin or Sinema opose as well, the President would have to nominate another candidate.
With Breyer expected to step down later this year (at the end of his term at the end of June), and the midterms in November, that could delay the process until after the elections. If the Senate swings to the GOP side, that would give them the upper hand in the pick (or at least get a more neutral pick).
Politics.