The President-elect is a term defined in the Constitution. It refers to that person who has been elected President of the United States but ho has not yet taken office.
Joe Biden's assumption of that title his a bit premature. The next president won't be elected until the first Monday, after the second Wednesday of December. That is when the electors meet to elect a president. Until then, Joe Biden is just some guy who may or may not be the president.
Many people are confused by this, which shows a distinct lack of knowledge about how we elect a president. Evidently, civics is not a thing in high school any more.
But, Sleepy Creepy Joe should know better. He's been a senator since Moses was a Lance-Corporal and a Vice President for eight years. His ignorance on this mater is appalling.