Thomas Edison in 1883 noticed that electrical current flowing through a light bulb's filament could make the wire so hot that electrons boiled off, sailing through the vacuum inside the bulb to a metal plate that had a positive charge. Because Edison didn't see any way the phenomenon would help him perfect the light bulb, he only made a notation of the effect, which he named after himself. The effect sat on the shelf until 1904, when a former Edison employee, inventor John Fleming, went to work for the Marconi Radio Company. For his first assignment, finding a better way to receive distant radio signals, Fleming began experimenting with the Edison effect. He discovered that radio waves passing through an airless tube created a varying direct current, which could be used with headphones to reproduce the sound carried by the waves. Fleming named it the oscillation valve and applied for a patent. Marconi, though, chose another, less expensive technology: a crystal wave detector.
Here's what I understand. The electrons did the action of boiling off and then sailed through the vacuum inside the bulb and so on and so forth. So, the only obstacle to the perfect understanding of that sentence I have is deciphering what exactly boiling off means in that context.