Designed by Reed Richards of the Fantastic Four, the Signal Device was originally used in early issues to alert other team members of impending trouble.  It was a specialized firearm that discharged a shell that exploded in midair, creating a fiery stylized number "4" also known as the Fantasti-Flare.  This was used by Richards as an emergency summons to the other members to report to the Baxter Building immediately. 

Later, the other members of the Fantastic Four took to carrying their own Signal Devices which they used to alert the other members of the team of their current location and that they were in need of help and to rally together.
 
However opponents of the Fantastic Four quickly realized the existence and usage of the Fantasti-Flare and used it for their own purposes.  The advent of more advanced communications devices have caused the Fantasti-Flare Signal to fall into disuse.
 
The Human Torch (Johnny Storm) is one of the few members who rarely employed this device and instead exerted control over his flames to create his own makeshift signal flare.  He also is one of the few who still employs this flare to this day, often as a signature or calling card denoting the presence of the Fantastic Four.