What is the fill flash technique?
I like to think of photographic lighting as "adding on" one light source on top of another.
Start with your "ambient" light. That's the light that's bouncing off all of the surfaces around you. Think of it as your base lighting. This is what you control by setting your exposure (ISO, shutter, aperture). A lot of dramatic photos are lit by reducing exposure so that the ambient light is (near) complete darkness.
Then, add your "key" light. This is the brightest and "most important" light. It's what lights your subject, from the angle you desire. You adjust the power on your light so that it gives you the correct exposure for the lit areas of your subject. Note that, unless you're lighting a flat/even surface from the same angle as your lens, you're going to get shadows on your subject. (Most of the time this is desirable.)
The fill light takes those shadows and reduces them: it "fills them in." It's a light that lights up the areas of shadow on your subject. (If it substantially lights the non-subject areas too, then it might as well just be ambient light.) By definition it has to be less bright on your subject than your key light. Often it will come from the same angle as the camera/lens, but that's not necessarily true.
You may not need a fill light, depending on what you're using for your key light & how stark you want your shadows to be.
"Fill Flash" is pretty much synonymous with "fill light", though it does imply using a "burst" light (flash/"strobe") rather than a "continuous" light (ie: your common household light bulb). But pretty much any light can be potentially used as a fill light.
Fill flash is a technique, rather than a piece of equipment.
Typically, you'd use some fill in flash if your scene is backlit, with the flash set to a relatively low power setting to lift the shadows.