A standardized test is an exam that is graded in the same way for every person that takes the exam. In other words, is is graded based on a standard and in a completely objective way.
In the US, there are different standardized tests. The great majority of people takes either the SAT and/or the ACT. . You can take only one of these, there’s no need to take both. International students, however, will also be asked to take the TOEFL, a standardized test that measures your English abilities
Standardized tests in the US are used to determine your capabilities in comparison to other students, since everyone is graded in the exact same way. This comparison will be one of the criteria for college admission teams to evaluate your application and make a decision about whether you are accepted or not.
It’s not a big deal, you can take both the SAT and ACT as many times as you want, and only your best score is the one that will be sent to colleges to evaluate you.