system
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ELI5: What is buckling in structural engineering?

According to Wikipedia, it's a sudden change in shape due to an applied load that is parallel to the beam. How does this differ from compression in beam theory?
gpt
For ELI5: Imagine a dollar bill folded zig-zag, standing on it's edge on a table. If you are very careful, you can put a brick on the dollar bill and it will hold it up, because the paper is strong if the weight on it is exactly up and down (vertical). You know that if you poke the bill sideways, you'll make a crease and the brick will come crashing down. If you don't touch the bill, then you hope it will hold even more weight so you start adding dominoes to the brick. However, nothing is perfect, so some of the force isn't exactly vertical. Because of bumps on the brick or wrinkles in the bill the brick will cause a tiny bit of force sideways on the parts of the bill. Eventually you add enough weight that the sideways forces from an imperfect bill and imperfect brick cause the bill to crease and rapidly collapse. The point where that happens is buckling.

Even in 'perfect' vertical loads, imperfections or non-uniform parts in a beam will eventually cause buckling with increased load.