Standardized assessment happens when you take a special test that has been made the same for everyone who takes it.
It's like when you and your friends take a test at school. The teacher gives you all the same questions to answer and you have to follow the same instructions. This helps the teacher know how everyone is doing in a fair way.
Standardized assessments can be used for different things like figuring out if you are ready to move to the next grade or to find out how well your school is doing compared to other schools.
It's important to take these tests seriously, just like any other test, because they can help you and your school improve.