According to NASA's article on the test, this is a big step toward sending a manned mission to Mars:
NASA successfully tested the first deep space RS-25 rocket engine for 500 seconds March 10, clearing a major milestone toward the next great era of space exploration. The next time rocket engine No. 2059 fires for that length of time, it will be carrying humans on their first deep-space mission in more than 45 years.
This is all part of a plan to send humans to Mars by 2030, which NASA has laid out in this beautifully illustrated image: