What is one tip for beginners?

Stop dropping pieces.

Stop being a doosh. ↑

actually, it's an excellent tip.

Hey dunce. I’ll spend twenty hours this week teaching chess to children. The advice I gave here is the advice that everyone below 2200 most needs.

Your reply, in contrast, has no value.

You’re still new to the site. 

I memorized 15 moves of sicilian theory but still hang 3 pieces a game! Why am I not improving???

Bullet is like that.

I was joking, since a lot of people around here explain that they're doing tactics, lessons, watching chess videos, etc. but not winning, which can usually be explained by not checking to see if a move is a blunder. I agree with ziryab that not hanging pieces is the best advice.

Its because they ignore the opponents threats and just look at there own threats

The single most important thing for an absolute beginner is to assimilate as many patterns as possible! (Strategic and tactical) How to do this? By solving tactical positions and by studying classics. The latter is ignored so much that kids these days manage a 2100 FIDE without knowing who Petrosian or Smyslov or Geller was. Well you can manage 2100-2200 FIDE. But to be really strong as Kramnik once pointed out, it's important to absorb the classics. 

So, till you are 1400-1500, give almost all of your 'chess time' to tactics. Maybe 10% to basic openings so that you don't get wiped out. When you achieve a ~1600 rating, it's time to go through the classics. Pick up a player you relate with the most. Go through his/her games. 

Good luck!

I think it comes with time, the more time you put in to chess the better it gets. But for example try looking more on your opponents threats. Also trying to memorize the position from the board to mind can be useful so you don't have to stare at the board to know pieces position. But I think this comes way later. But in start just play and analyse the games after, the board vision will be better over time.

Oh and I've learnt it the hard way, i'll share it here. When you solve puzzles, just guessing the first move may fetch you puzzle rating but it means nothing until you've seen the entire sequence in your head before you start entering the moves. 

So refrain from immediate gratification by impulsively guessing the move. Calculate systematically. See the sequence, identify opponent's resources. Evaluate the end position. If you like it, play it. If not, look for more. 

take classes

Saying "stop dropping pieces" is not a tip, its more like a fundamental baseline. Its like saying new drivers "keep your eyes on the road". oh...really?. I'm pretty sure a new player who got here wants more specific advice than that.
Anyway, I thought it was meant as an snob contemptuous condescending remark, like "play better noob", but turns out you actually meant it.
So, I stand corrected, sorry. I am the doosh!.

I offered other advice when the thread was new: https://www.chess.com/forum/view/for-beginners/what-is-one-tip-for-beginners-69660927?page=2#comment-69671981

I mean that, too.

Learn to think in terms of fundamentals contacts--how the pieces interact. Not offensive contacts (threatening capture), restrictive contacts (protection and pins). Yuri Averbakh, Chess Tactics for Advanced Players develops terminology and theory related to contacts. This Kindle book applies Averbakh's ideas with elementary exercises for beginners. https://www.amazon.com/gp/product/B06XKG1VZD/

Bruce Pandolfini, Beginning Chess also has very simple exercises with few pieces. I think it is useful.

My journey in chess took its first big jump from clueless beginner (I was far worse than most players rated 300 here) to being able to beat all my friends several decades ago when I started studying miniatures (short games) in Irving Chernev, The 1000 Best Short Games of Chess (1955). It is out of print. But, you can find similar games at https://www.chess.com/forum/view/game-showcase/post-your-best-miniatures-here