The stats seem intimidating at first but they're really not as complex as they might seem. I'll try to give an overview of the most important ones to know.

For hitting: Back in the day, the only hitting stats anyone really paid attention to were batting average, RBIs (runs batted in), and home runs, with a .300 batting average and 100 RBIs considered the cutoffs for what makes a good hitter. When Billy Beane and Moneyball came about, through the use of analytics they were able to shift the focus from these flawed stats to ones that more accurately determine a player's value. For example, batting average is flawed because it treats all hits as the same, whereas a homerun is much more valuable than a single. A .300 hitter who only hits singles is much less valuable than a .250 hitter who can hit doubles, triples, and home runs. RBIs are also very flawed because it is very team dependent. Hitters in better lineups will get more opportunities to drive in runs, and great hitters with poor teammates will be unfairly penalized. Moneyball put a much heavier focus on on base percentage, because hitters who get on base are creating more scoring opportunities and making fewer outs.

If I had to pick one offensive stat to know, it would be OPS. OPS is just a simple addition of a hitter's on base percentage and their slugging percentage (slugging is the average number of bases a hitter gets per at bat, with a .500 slugging percentage typically being the mark of a strong power hitter). A .750 OPS is average, .850 is All-Star territory, and .900 and above are elite hitters. Even if OPS is the only stat you know, you'll more or less know who all the best and worst hitters are.

If you want a stat that's a bit more advanced, wRC+ is the best one. It stands for weighted runs created, and it attempts to account for the current year's offensive environment and adjust for ballpark effects. Some ballparks like Coors are a hitters' paradise, so a .900 OPS there is not as impressive as a .900 OPS in a ballpark that suppresses offense like Oracle. The calculations of wRC+ might be complex, but the way to understand it is not. A 100 wRC+ is the average hitter, while a 110 wRC+ is 10% better than average and a 90 wRC+ is 10% worse than average. When you see someone like Mike Trout or Juan Soto sporting a 200 wRC+, that means they were twice as good as the average hitter that year.

Pitching: Traditionally the most important pitching stats are ERA (the average number of runs a pitcher allows per nine innings) and wins. The analytics here are a bit more complex than with hitting. Virtually no one these days takes pitcher wins seriously anymore, as it's even more team dependent than RBIs, and a pitcher can have a dominant game but take the loss if his offense doesn't support him, or conversely pitch like trash and still get the win because his offense had a great night. ERA is still important, but it's lost significance because it is dependent on many things outside of a pitcher's control, such as the quality of the defense behind the pitcher. This led to the creation of FIP (fielding independent pitching), which is calculated using strikeouts, walks and home runs allowed, and is supposed to reflect what a pitchers' ERA "should have been" if you take the defense out of the equation. FIP is a better predictor of future ERA than ERA itself, but it still has it's own flaws, as pitchers who have the ability to induce weak contact are able to "outperform" their FIPS.

For pitching, I'd stick with ERA for now, but looking at FIP can still be helpful if there's a large divergence between the two numbers, as it most likely indicates unsustainable performance.

Fielding: I'd forget about fielding for now, as even though defensive stats are still improving every year, they're still a bit of a black box and don't tend to be reliable until you have multiple seasons worth of data, and even then there are confounding variables. The only thing I'd note here is that the more valuable defensive positions such as shortstop, catcher, and center field have a much lower acceptable bar for offensive performance. A slick fielding shortstop with an .800 OPS is likely to be just as valuable if not more so than a first baseman with a .900 OPS, because virtually any major leaguer can be taught to play first base, while it's much more difficult to find someone with the right athleticism and instincts to handle shortstop.