When you do something rashly, you act without stopping to think things through. Don't rashly promise your friend that you'll bake three dozen cookies for her party without making sure you have time to do it!
Careful, thoughtful people tend not to act or speak rashly; instead, they consider the effects of their actions. A decision made rashly can have unintended consequences, like when you rashly decided to ride your bike to your grandparents' house, not stopping to think about the fact that they live 500 miles away. Rashly and the adjective rash come from a root meaning "flash of lightning." Make decisions as quick as lightning, and you make them rashly.