Cohesion means you make sure that the parts that belong together are close to each other. 💥 Bam! Shortest blog post ever!
On a more serious note let's talk about why this is important. If you would have asked me ten years ago I probably would have known about principles like DRY but not about cohesion and its evil twin coupling. I asked my team. Everyone knew and could explain the DRY principle. Some had heard about cohesion but no one could come up with a good explanation. Why is the one principle easier to learn and understand than the other? I think principles like DRY are easier to apply because they come with a set of distinct rules. If you find the same code block multiple times then you're code probably isn't DRY. In contrast, cohesion does not have such a set of rules you can follow. You rather need something we call "Fingerspitzengefühl" in Germany.