Assumed incumbency is a big fancy term that refers to something that happens when someone who is already doing a job gets to keep doing it without any further action needed.
Let's say you’re playing a game of tag where one person is "it" and they chase the other players. If the game goes on for a long time, the person who is "it" might get tired and want to stop playing. But if they just say "I quit!" and walk away, the game can't continue.
So instead, the person who is "it" might say "I need a break, can someone else be it for a little bit?" This way the game can keep going and everyone can keep having fun.
Assumed incumbency works kind of like that. Sometimes when an important job like being the leader of a company or a country comes up for a vote, the current leader might say "I want to keep doing this, but I don't want to have to campaign and run for reelection again."
If enough people agree that the leader is doing a good job and should get to keep leading without having another big election, then they might say "Okay, we'll just assume that you are still the leader until we decide otherwise." This means the leader gets to keep doing their job without having to worry about winning another election.
Overall, assumed incumbency is just a way to make sure that really important jobs can keep getting done without causing too much disruption or chaos.