Actually 4e introduced it and 5e repurposed it.
It means 6-8 combats with one or two short rests in between. Which the DMG explains. It doesn’t explain that combats are expected to take about 3 rounds, and it probably should, but all the rest of these assumptions were explicitly stated. A lot of people just… didn’t like those assumptions and decide to ignore them, and then complain that the game wasn’t balanced.
It introduced in the sense that it gave a name to something and the rules were built with the concept in mind. It predades 4E as an aspect that affected D&D games before, since it has some game reosurces that are limited by uses per day - spells, rage, or whatever. The less combats you have per day, the more domineering those "dailies" become, the stronger opposition they can face.
I think 3E (or 3.5) also had some sort of mentioning of encounters per day when it came to dealing with challenge ratings and encounter levels.
4E made it a stronger point to design aspects of the game around the notion that this effect simply exists. It kinda reigned in the power of those daily powers, so they were a little less powerful, but also gave every class such abilities, so that diverging from some baseline assumption of encounters per day wouldn't benefit only some classes.
In my own games, we rarely managed to reach the baseline of combat encounters per day that D&D 3rd Edtion, 4th Edition or 5E Edition assumed. With 4E I at least knew running them that my party could handle more powerful opposition (though I still underestimated them often), and no class would suffer from it.
I think if you really want those daily (or weekly or whatever) resources in game, you should be transparent about what your baseline is, and what kind of issues could arise (ideally also with ways to fix them.)