The term “Widow’s Bite” can just refer to Natasha’s special gauntlets, that have a variety of wacky comic book capabilities that vary from writer to writer depending on how wacky comic book they want to get with it, in the manner of Batman’s utility belt that sometimes contains shark repellent. The gauntlets always have some kind of electrical charge (“Widow’s Sting”), basically a turbo-taser that can be set to lethal blast mode when stuff really needs it. They also have a grappling hook (“Widow’s Line”) that lets Natasha swing around on rooftops.
Sometimes they’ve got other stuff built in— trackers, a garrote, or yet more funny spy devices—
“I call it the Widow’s Kiss. Instant knock-out drug, pioneered by Russian Spetsnaz.”
Sometimes “Widow’s Bite” is used to describe the primary offensive capabilities of her gauntlets in specific.
Natasha: It’s called a Widow’s Bite. Percussive energy blasts. These guys are going to feel it for at least a month.
Nick: Kiss… bite… you’ve got this whole oral thing going on, don’t you?
Natasha: Part of me wants to nudge him off the back of this snowmobile.
Most of the time the weapon just knocks dudes out, but it can be more deadly when Natasha/the story needs it to be. This is also a comic book weapon, so it works equally well on rampaging mutant dinosaurs as it does on enemy agents— the blasts are generally a cooler visual, imo, than mundane guns are. It’s fair to assume that she gets them upgraded pretty often, explaining why they can work a bit different from comic to comic.
Scans from Black Widow #7, by Duane Swierczynski and Manuel Garcia.