I don’t watch baseball but for some reason I’ve seen people complaining about it on social media.
Typically players are fairly symmetrically placed out in the field. Coaches sometimes ‘shift’ the position of players in the field towards the areas where a batter historically hits which supposedly gives them a higher chance of catching the ball and making an out.
The word ‘fans’ is short for fanatics, which answers your second question.
The shift in MLB is when the defensive players “shift” from their normal positions to more of one side of the field. They usually do this because a batter will have a tendency to hit on one side or not. By shifting coverage to the side they hit to, the defense has more of a chance to get the ball faster.
The problem that people seem to have is that if lowers base hits. Which means there are less base runners. Which means less scoring.
So they implemented the ban. This is to hopefully have more base hits and maybe make baseball more exciting to watch.
The shift is when you shift your defensive players to one side of the field. A standard defensive alignment looks like [this](https://img.mlbstatic.com/mlb-images/image/private/mlb/nf7jyuk3ynumcnqvsmgh) a shift might look like [this](https://img.mlbstatic.com/mlb-images/image/private/mlb/ldmedhvlznyovl5gngta). The reason you would shift is that no hitter hits to all fields equally, and a number of hitters hit very heavily to one field. So for example if you have a left handed hitter who pulls the ball you would move your fielders to the right side of the field, like in the image, to have better defensive coverage.
The shift has existed forever, but with the invention of modern analytics and data the use of the shift has exploded. In 2017 a shift was used in roughly 12.5% of all plate appearances, in 2021 it was 31%. Left handed hitters saw a shift in *52%* of all plate appearances.
The issue is that the shift lowers the amount of base hits, which lowers scoring. Left handed hitters batting average on balls in play for ground balls is *52* points lower against the shift than without it. 52 points is a huge difference.
The shift has meant that ground balls are essentially guaranteed outs, especially for lefties. This has led to an increase in what is called “three true outcome” players. Three true outcome hitters are players who tend to hit a home run, strike out, or walk. For example in 2021 Joey Gallo had 99 hits of which 38 were home runs; almost 60% of his plate appearances ended in a strikeout, walk, or home run.
Baseball is trying find ways to make the game faster, the average MLB game is something like 3:05 long, because the MLB is hemorrhaging fans. They hope that banning the shift will make games more exciting. They hope other changes like the pitch clock will speed games up, it appears that the pitch clock could cut about 30 minutes from a game.
The reason people are upset about the shift is that it has existed for a long time and they think players should just adjust to it and they generally just dislike changes in the rules. Baseball has been around a long time, and has some fans who are pretty old school.