As seems to be the case with everything to do with american elections, it's a bit of a mess.
Two senate seats were up for grabs in Georgia in the November election. David Perdue's term was coming to an end, so an election was needed. It officially ended on Jan 3rd, so he's out of the senate. This senate seat is now vacant until the winner is sworn in.
Kelly Loefler has actually never won an election - the previous senator resigned due to ill health at the start of 2020. His term was supposed to run till 2023, but in Georgia the rules are that a replacement can be named, and sit in the Senate, but must be confirmed by a special election. This special election was scheduled for November so they could combine it with everything else, which make sense. As a result, Kelly Loefler will remain a senator for Georgia until her successor is confirmed/sworn in.
Both the senate elections are run-offs. This happens when neither gained a majority. In November both races ended with candidates on 49% each, as a tiny minority voted for independent candidates. With neither of the senate races seeing a candidate get 50% of the vote, we moved to this runoff scenario where it's just the two candidates for each seat, so someone must get 50% or more.