HBr adds to alkenes to create alkyl halides. A good way to think of the reaction is that the pi bond of the alkene acts as a weak nucleophile and reacts with the electrophilic proton of HBr. Alternatively, you can view the first step of the reaction as the protonation of the pi bond. These are really just two ways to think about the same event. Either way, a carbocation intermediate is formed along with the bromide anion during the initial step of the reaction.

In unsymmetrical alkenes, the more stable of the two possible carbocations will form predominantly. Carbocations are stabilized by either the presence of alkyl groups (hyperconjugation), adjacent pi bonds (resonance), or atoms with a lone pair (resonance) attached to the cationic carbon atom. One definition of an intermediate (as opposed to a transition state) is that it has a measurable lifetime during the course of the reaction. In the movie, the carbocation intermediate is the species present following initial protonation of the alkene, and before the new bond is made with the bromide ion.

In the second step of the reaction, the bromide acts as a nucleophile and reacts with the strongly electrophilic carbocation to give the product alkyl halide. In this movie, a new bromide ion reacts with the carbocation, not the one from the original H-Br molecule. This was an arbitrary choice, as in reality, the H and Br atoms transferred to a given alkene might indeed derive from the same H-Br molecule. Keep in mind that these animations were prepared without added sovlent, which has a large influence on the details of proton transfers and reactions between charged species.

The overall transformation of HBr addition involves breaking the carbon-carbon pi bond and creation of new carbon hydrogen and carbon bromine sigma bonds. The carbon atoms change hybridization state from sp2 to sp3. Because the bromine ends up on the carbon that was positively charged in the carbocation intermediate and alkyl groups stabilize adjacent carbocations (hyperconjugation), the bromine ends up on the more highly substituted carbon (middle carbon) of the original alkene. This regiochemistry is referred to as Markovnikov's rule.