It means when it starts growing flowers which are unwanted, instead of leaves. It happens when conditions are hot and/or dry, or because of not watering enough or watering at the wrong time of day (i.e. when it is too hot at midday and the water evaporates away instead of soaking into the ground).

Try this:
http://www.gardeningknowhow.com/vegetable/what-is-bolting-what-it-means-when-a-plant-bolts.htm