Mechanic Q: CHECK ENGINE LIGHT ON?

2000 WINDSTAR 46000 MILES 3.8 ERROR CODE HO2S21 PLEASE TELL ME WHAT THIS CODE MEANS AND HOW TO TEST.

Q: check engine light code ho2s21 bank 2 sensor 1?

PLEASE HELP ME LOCATE WERE SENSOR IS AND WHAT THE TEST VALUES ARE SO I CAN CHECK