Evildela:
Aprila vetero -- trompa aero.
Aprils weather --- fake air

What’s that meant to mean? When would it be used. What situation would it be used in.

It's a northern hemisphere thing, can't be used in Australia! I think it's supposed to mean that winter is still lingering around and you can get some warm days in April which make it seem like the cold weather is behind you, but isn't really. The weather is deceitfully warm, you might say. More cold days are likely! So if you're strolling in Oslo one warm April day, and your friend remaks that la vetero estas varmeta, you grin and hit 'em back with that.

That’s exactly what I mean, you needed a paragraph to explain what that proverb meant! I'd love to be able to use all of the native Esperanto proverbs, but I have no idea what most of them even mean.

Septembra vetero -- trompa aero is the correct form.

Aprila vetero -- trompa aero is clearly a typo. Especially at the moment in Adelaide.

(mi nur ŝercas!)

trompa doesn't really mean fake, it means that deceitful. Or "tricky". trompi means to trick someone.

So April air tricks you.

The meaning of the proverb was clear to me actually, and most Esperanto proverbs in the proverbaro make sense to me (I can understand their intent) if I think about them for a moment. However it requires a mastery of Esperanto vocabulary; correct understanding of "trompi" is crucial in this case.

I think that even in English, proverbs are not always immediately obvious in meaning. They are obvious to us because we grew up with them but it might not be obvious to someone else. If you had grown up with these Esperanto proverbs, I have no doubt that their meanings would be obvious.

The Esperanto proverb about April reminds me of the English proverb about March - March comes in like a lion and goes out like a lamb.

I think a proverb like "the apple doesn't fall far from the tree" might require a little explanation to someone hearing it for the first time, or at the very least, the person would have to think about it for a second to reason it out. And yes, it might require a sentence or two to explain it for someone who hadn't heard it before.

My one-sentence explanation of aprila vetero, trompa aero - It refers to the fact that the weather can be very changeable in April, because it can fool you into thinking that winter is entirely gone, although it isn't.

(personally, I think that if this proverb were written with reference to my region, it would be talking about March, and not April. But most of Europe is a bit cooler than where I live)