Justificationism is like playing hide-and-seek with a grown-up. When you hide, the grown-up tries to find you by using clues like footprints or sounds you make. They keep searching until they find you and then they say, "I knew you were there because I found you. I am right!"
In the same way, people use justificationism to try and find out if their beliefs are true or not. They use clues or evidence to support their ideas and when they find enough proof, they believe they are right.
Justificationism is like a game of finding clues to prove that you are right. The more clues you find, the more justified you feel in believing what you think is true.