Imagine that you are in a classroom and you have just finished a test. After the test, the teacher goes through everyone's papers and grades them. Some students did really well and some didn't do so good. This is kind of like what the last judgment is like.
In some religions, it is believed that after a person dies, they will stand before a big, powerful judge called God. God will look at all of the things that person did during their life, both good and bad. Just like the teacher grading papers, God will judge each person and decide if they are going to heaven or hell.
For Christians, the last judgment is described in the Bible in the book of Matthew. It says that Jesus will sit on a throne and all of the people who have ever lived will be divided into two groups. One group will be the "sheep," who are the people who helped others, showed kindness, and followed Jesus. The other group will be the "goats," who didn't help others and didn't follow Jesus.
So, the last judgment is a big, final decision made by God about where people will spend the rest of eternity. It is a way of separating people who have followed God's teachings from those who haven't.