Markovnikov's rule and the carbocation
When a protic acid such as HBr adds across an asymmetric alkene, Markovnikov's rule predicts the hydrogen attaches to the carbon that already has more hydrogens, placing the halide on the more substituted carbon. The reason is the intermediate carbocation: the reaction proceeds through the more stable carbocation, which is the more substituted one because alkyl groups donate electron density.
The classic shorthand, the rich get richer, summarizes the outcome but the carbocation stability argument is the actual driver.
Anti-Markovnikov exceptions
Adding HBr in the presence of peroxides reverses the regiochemistry. This radical mechanism, the peroxide effect described by Kharasch, proceeds through the more stable radical rather than a cation, so the bromine ends up on the less substituted carbon. Hydroboration-oxidation also gives anti-Markovnikov alcohols because boron adds to the less hindered carbon.