Almost all modern languages rely on word order to convey meaning. Were I to say, “the ball fell,” you’d know what I meant. I’m talking about a ball, and something it did, which was to fall. If I said, “the fell ball,” I’d be conveying an entirely different meaning, that of a ball which is vicious and cruel, turning “fell” from a verb into an adjective simply by placement. But this word order situation hasn’t always been the norm.
High Latin, for example, included a feature called declension. To vastly oversimplify it, declension attaches a suffix of some kind to indicate the purpose of a word in the sentence, rather than relying on word order. I might say, “the ball-a fell-o,” or “the fell-o ball-a,” and they’d carry the same meaning, presuming “a” denotes the subject while “o” indicates the verb.
Languages that use declension are weird for a modern speaker whose language doesn’t use it.
So you might wonder why the Romantic languages, modern descendants of Latin, don’t feature declension. French, Spanish, Catalan, Portuguese, Italian and the like are all “missing” declension. That’s because our Romantic languages descending from Vulgar Latin, the less-formal languages of the masses (and slaves), rather than the form High Latin that your well-to-do emperor-types would use.
And declension isn’t dead. Many modern languages include at least a form of it, including Arabic, Finnish, Turkish, Hungarian, Russian and Ukrainian, just to name a few. Declension itself comes in different forms, too, sometimes relying on a tonal inflection instead of a suffix, and often occurring in different contexts.