PSAT is a preliminary test and is meant to give you an idea of how well you’ll score on the SAT. The PSAT score does not count for college admission.
The SAT Reasoning tests math has algebra II questions that are absent from the PSAT math test. However, these questions are few and the overall difficulty of SAT math is not much higher than that of SAT reasoning. The SAT Subject tests level 1 and level 2 have more advanced questions than PSAT.
If you are a junior, a high PSAT score can help you to be considered for the National Merit Scholarship Program.