HBr hits propene, and bromine lands on the middle carbon, never the end.
On paper, two products look possible. Real flasks make almost only one.
First, the pi bond grabs H plus. The hydrogen joins the carbon already holding more hydrogens.
That leaves a secondary carbocation, stabilized by neighboring carbons, far more stable than a primary one.
Then bromide attacks the positive carbon. So H goes to the carbon with more hydrogens; bromine goes to the more substituted one.