Markovnikov Rule predicts the regiochemistry of HX addition to unsymmetrically substituted alkenes.

The halide component of HX bonds preferentially at the more highly substituted carbon, whereas the hydrogen prefers the carbon which already contains more hydrogens.

The mechanism as shown below,  the proton adds first to the carbon-carbon double bond. The carbon bearing more substituents forms a more stable carbenium ion; attack of bromide ion follows in a second step:

Radical reactions require an initiation step. In this example, a bromo radical is formed.