Psychometrika is like a game that helps grown-ups understand how smart they are!
First, some grown-ups make up questions that are tricky and measure how well someone knows something. They call these questions "tests".
Next, other grown-ups take the tests and answer the questions. Then, the first grown-ups use some fancy math to figure out how well the test takers did. They can tell us how good someone is at something or if they need more practice.
It's kind of like playing a game and getting a score. Psychometrika helps grown-ups understand how well they did in the game of learning.