This is an arugula flower. We plant arugula for its greens that we harvest in cool weather. Once we have hot weather like the near 90's of last weekend, the arugula bolts - that is it sends up a stem with flowers to set seed.
We harvest the stems for the flowers. They are a great addition to tossed green salads as they retain the delightful peppery flavor of arugula. After adding the salad dressing top with these or other edible flowers - chives, pansies, violets, nasturtiums, etc.