To take measures to check the quality, performance, or reliability of something, especially before putting it into widespread use or practice. The engineers will test the new software to ensure it functions correctly.
To give someone a set of questions, problems, or exercises to measure their knowledge, ability, or experience. The teacher will test the students on their understanding of the material covered in class.
To reveal the strengths or capabilities of someone or something by putting them under strain or pressure. The marathon will test the runners' endurance and determination.