What it costs you in storage
Bursts are the best-disguised weight in a photo library, because the cost is hidden behind one thumbnail. A handful of bursts at a birthday or a football match can be several hundred full-size files, and you will keep one frame from each. Multiply that across a few years of kids, pets and concerts and it is one of the largest single categories in a big library.
How you shoot one (often by accident)
On iPhone 11 and later, holding the shutter records video instead — a burst is sliding the shutter button to the left. You can also hold the volume-up button if Settings › Camera › Use Volume Up for Burst is on, which is where a lot of unintended bursts come from.
What you can do about it in iOS
Every burst you have ever shot is collected in Photos › Albums › Media Types › Bursts. Open a stack, tap Select… at the bottom, tap the frames worth keeping, then Done — iOS offers to keep only the frames you picked and delete the rest, or to keep everything. What it deletes goes to Recently Deleted for 30 days like anything else.
It works, and on a library with hundreds of stacks it is a long evening: every stack has to be opened, judged frame by frame at thumbnail size, and closed.
Burst, or just similar?
A burst is a formal thing — iOS marks it, stacks it and files it in one album. The three separate tries at the same group shot are not a burst; they are near-duplicates, they sit loose in your timeline, and no album gathers them. Both cost the same per frame.
Kept groups both on the device and suggests the sharpest frame from each run, and you confirm or overrule it one swipe at a time.