Every language has its idioms, which lends color to how we express ourselves both in speaking and in writing. Because idioms express more than their literal meaning, to non-native speakers they sound like a bizarre string of unrelated words. Like being on the ball—only literally possible for circus acts but figuratively speaking, it's a great way to imply someone can react quickly.
Linguists used to believe that idioms were arbitrary and it was impossible to guess their meaning. But many idioms can be explained, like put something on the back burner, which obviously comes from cooking.
Looking at an idiom's origins helps unlock its meaning. For example, both English and Dutch are rich in expressions connected to sailing, hardly surprising given their seafaring history. The tricky part is that most idioms are inherently tied to culture and don't always translate—which is why the English drink in order to have "Dutch courage," something the Dutch know nothing about.
There's huge fun to be had in learning foreign idioms because they work like a window into a culture's soul. Like kummerspeck (German), a word used to express extra weight gain due to emotional eating; or cavoli riscaldati (Italian), which literally means "reheated cabbage" or what happens when you try to revive an unworkable relationship.
A great way to learn idioms is by listening to music. Look at song lyrics, and you have a ready-made context for understanding an idiom. Idioms come in handy in everyday life and the key to sounding more fluent is to use them in writing or speaking.