The answer is actually right in front of you — on your keyboard. On a standard QWERTY keyboard, “asdf” are the first four letters in the home row, the term that refers to certain keys in the center row of a keyboard or typewriter.
When we learn to touch type, one of the first repetitive exercises we do involves the keys in the home row. Touch typing is typing without looking at the keyboard. A touch typist places eight fingers on the home keys and then reaches for the other keys from this position.
A skilled touch typist makes few typos, which is short for typographical errors. Originally the term referred to the manual typesetting of printed material. But now it is most often used to talk about what happens as a result of hunting and pecking, or thumbing.
Let’s return to that wacky string of letters: “asdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdf.”