How our government works, here in the USA.
What, in your opinion, needs to be better understood by more people?
That we can lessen the gaps in opinions by instead of reacting to what people say, and listening to them and responding in a less emotional and more intelligent way.
That even one's least actions affect others, either directly or indirectly, favorably or not, intentionally or not. The greater the action, the more this is so. Better reasoning coupled with any action would lead to a better world.
Government workers
British "Dark Ages" history. Or otherwise, astro-physics. Ornithology, perhaps. OK, lots of things (and it is not a coincidence that they mirror my interests).
Why 500 Electoral votes were able to elect a man who had less votes than his opponent.
Money, loans, banks and the Federal Reserve system.
Other people, values and belief systems, Science, Abortion, sexual assault, global warming, basically everything.
They should know that other living things are born to live happily not to serve humans they should respect their living value and also they should understand that earth is not only for humans.
Life.
That as far as we know, this LIFE is all there is.
If more people fully understood that this is it . . . I personally believe they would make more of it. They would aspire to be or do more, they would treat the planet better with the understanding that the previous generation tried to do the same for them.
They would embrace the need to get along since it is about the species and not the differences in how we live our lives.