The Reichstag Fire was significant because it gave Adolf Hitler a pretext that allowed him to take more dictatorial powers over Germany.
Hitler became the Chancellor of Germany on January 30, 1933. At that point, Germany was still a relatively democratic country. The Nazi Party did not yet have the kind of complete control of the country that it would later have. Hitler wanted to subvert the democratic system so that he could eventually rule as a dictator. The Reichstag Fire allowed him to move in this direction.
The person who set the fire in the Reichstag was a Dutch communist. He was also somewhat mentally handicapped. The Nazis used the fact that he was a communist to “prove” that there was a communist conspiracy within Germany that aimed to take power in the country. Using the fear of this conspiracy, Hitler was able (partly because he was able to ban all communist members) to get the Reichstag to declare an emergency. The enabling act that the Reichstag passed gave Hitler the power to rule by decree.
Thus, the Reichstag Fire gave Hitler the perfect excuse to take greater power over the German system. The fire is significant as a major step along the way to total Nazi domination of Germany.