The PSAT/NMSQT doesn’t require you to learn unusual or difficult vocabulary words. The words you’ll be asked about come up often in college-level reading and in professional life.
In addition, the PSAT/NMSQT also doesn’t ask you to define a word without any context to guide you. All of the words you’ll be asked about appear in the context of reading passages, so you can use context clues to guide you to the best answer.
If you build your vocabulary by learning the meaning and usage of words that appear most often in college-level assignments, you’ll have a much easier time with the test. In particular, you won't have to spend time guessing from context what a word might mean. If you recognize vocabulary words on sight, you'll read passages faster and with greater confidence.