What does this mean and why is it a problem?

It means that the cake is growing, but your share is getting smaller. Companies declare record profits and celebrate by mass firing people. People with big investments are never at risk of losing money due to inflation. Meanwhile, workers' salaries are in a constant struggle against inflation and cost of living.

There's huge amounts of money circulating around, but most of it ends up in the pockets of very few people. To ensure that even more money ends up in their pockets, they invest in new venues that will get more of your money for themselves. Because a very small number of people can simply buy up "everything", and usually do so for pure speculation, prices rise faster than your salary. Rent and home prices keep going up because of this, people that actually want and need a home don't have the means to buy them, but a single asshole with money can buy a lot of stuff, drive up prices and fuck everyone who can't pay.

If you don't see a problem with wealth getting more and more concentrated in fewer and fewer hands as time passes, you probably want few people to effectively control the world.

But they don’t exactly have the power of Mansa Musa.

They do. Anyone with over 100 million dollars laying around could easily crash some local economies, maybe not in the USA, but definitely in a number of developing countries' cities.