When you home in on something, you focus your attention on it. Your brother who is always reading probably homes in on bookstores when he visits a new place.
When you use the phrase home in, picture a sharpshooter aiming her gun, or a guided missile heading straight for its target. When you home in, you move toward or focus on a specific point, the way a homing pigeon homes in on the exact spot it started from, even after traveling hundreds of miles. The phrase home in was first used during World War II, and became popular in the period following the war.