In the gardening world, bolting refers to when the plant (usually vegetables and herbs) puts all its energy into flower and seed based growth instead of the expected leaf growth.

Plants are for the most part triggered into bolting because of hot weather.  Once temperatures get to a point where the plant will have difficulty surviving it will stop leaf growth and try to produce flowers and seeds as quickly as possible.  It’s easy to tell when a plant has bolted, because it will send out long shoots topped by flowers that will eventually contain seeds.
Some plants that are known for bolting are basil, broccoli, cabbage, cilantro, lettuce and spinach.

Once a plant has fully bolted, the plant is normally inedible. The plant’s entire energy reserve is focused on producing the seeds, so the rest of the plant tends to become tough and woody as well as tasteless or even bitter.

Bolting can be prevented by either planting the plant early in the spring so that it grows during the late spring or plant late in the summer so that it grows during early fall. You can also add mulch and ground cover to the ground, as well as watering regularly in order to keep the soil temperature down.  As well, some strains can tolerate heat better than others, making them more bolt resistant.