Boom! Bam! Pow! A crash is when two things collide, or smash, into each other. Crash and smash have similar meanings and sounds.
You can start to worry when you hear the word crash. Many people die in car and airplane crashes every year. Other crashes are less physical yet just as bad. In a stock-market crash, a lot of people lose a lot of money. You can also use crash as a verb. You'd better not crash your bike into the coffee shop window! If your computer crashes, you will probably lose all the data in it.