What does buckling mean?

Taking a coil spring as an example, a spring with a large aspect ratio (free length/center diameter) will experience a buckling (body bending) phenomenon when subjected to an axial load.
This is called buckling.
According to JIS, this aspect ratio should be from 0.8 to 4.

By the way, some springs that utilize this buckling phenomenon are used in keyboards. (“Buckling spring keyboard”)
This makes it possible to create an exquisite key touch.