First of all, the suggestion Ralph made for rescue was to build a fire in a place for the smoke to easily be seen by ships. This means they would have to build the fire away from where they currently were because they would have to climb up to the highest point.
The boys respond with such excitement that they race up the hill and build a blazing fire. However, they were too excited. Everyone went and got branches and fuel for the fire. With everyone's contributions, the fire took off quickly and accidentally burnt half the mountain. This situation was important because it demonstrated what acting too quickly could do. It also showed how quickly an idea's importance can leave. These guys were excited in the original moment. But, as time went on, you will see in later chapters they do not remain excited to go and tend the fire.