Halides are the elements in the periodic chart in the same column as chlorine. They are an electron acceptor. They tend to form stable compounds with elements in the column along with hydrogen and zinc and sodium, which can easily be coaxed to lend an electron.
So for instance, zinc chloride is probably the most popular "halide" containing compound that is added to flux. It happens to dissociate at soldering temps, and the chloride ends up taking an electron from hydrogen containing compounds (like water and other organic stuff in the flux) to make acid. This is pretty much like making hydrochloric acid by heating zinc chloride. (I detest industry pseudo scientific buzz words like "activator," because they don't mean anything.)
The problem with halides like chlorine is they imbed in iron and steel, and in conjunction with humidity in the air, they eventually cause red rust to form. The tip of your iron is made of iron. Halides are not good for your iron, or for any iron contacts like battery terminals or the like. They can't be washed off. They have to be etched or abraded in order to be removed. Since your iron tip is completely covered in solder and/or chrome at all times, it will be okish. But the battery terminal won't... unless you completely coat all of the iron with solder. Copper and tin and w/e is in most electrical component leads is generally ok with halides, even though there a small amount of iron in these things (esp surface mount parts; I have never found a surface mount component with leads that aren't at least slightly attracted to a magnet). This is because the iron content is fairly low, AND they are pretinned, completely covering the surface from exposure to moisture and oxygen. It's mostly the high iron content stuff like battery terminals where halides are a big problem.
These flux residues usually have to be washed in water. Ionic compound like zinc chloride dissolve in water.