When you zoom in on something, you take a closer look at it. You might zoom in on one particularly beautiful sentence in a book you're reading, for example, reading it slowly a few times.
To zoom in is to concentrate or focus on a small detail or point. You probably tend to zoom in on things that interest you, whether they're particular colors in oil paintings or strange looking birds perched in trees. The phrase comes from what a photographer does with a telephoto or zoom lens. She can adjust the camera's focus to see small or far away things up close: in other words, she zooms in on them.