To me it means consistency from round to round, box to box, lot to lot. The exact ammunition you buy should be the same, from a box/lot purchased this week, to another purchased in a year.
Some of it is, with excellent QC from box to box.
But much of it it not so. The powder can be dirty, leaving a lot of residue build up very quickly. Or the lot numbers of the powder used are not very equal in consistency. Or the manufacturer has obvious loose standards from cartridge to cartridge when it comes to the exact amount of powder in each loaded cartridge.
Visibly, sometimes some HPs or SPs can have visible nicks and dings all over the exposed lead nose. No 2 bullets will have the same overall shape/contour, and I believe this will have effect on the accuracy of the ammunition, especially at 50 yards and beyond. Or the weight of the bullets, again not a lot of consistency.
For plinking, cheap ammunition is usually perfectly good, but when ever you find yourself taking careful aim and trying for the 10 ring, or looking for the tightest group possible at 100 yards, or looking for maximum pistol performance in some way, cheap ammunition isn't always a good deal. And by comparison expensive ammunition sometimes isn't worth it either.
As you shoot, try new ammunition, even the same bullet weight/type, but from different manufacturers. Each gun is individual, and what works well for you in your 9mm might not in mine.
At the end of the day, the highest quality possible, the tightest consistency possible from round to round, is found only at the end of a loading bench.