electrophilic addition
Electrophilic addition is the signature reaction of alkenes and alkynes and the beating heart of this whole field. Picture the pi bond as a soft cushion of exposed electrons. A reagent that is hungry for electrons — an electrophile — is drawn to that cushion, the pi electrons reach out and grab it, and in the process the double bond opens up so that two new groups end up bonded across what used to be the C=C. A double bond becomes a single bond, and two carbons each gain a new partner.
The mechanism almost always runs in two steps. First, the pi electrons attack the electrophile (say a proton from HBr), forming a new bond and leaving the other carbon short of electrons — a carbocation. Second, a nucleophile (the bromide left over) swoops in and bonds to that positive carbon. Curved arrows tell the story: the first arrow goes from the pi bond to the proton, the second from the bromide's lone pair to the cation. Crucially, those arrows track moving electron pairs, not atoms.
Because the intermediate is a carbocation, everything that stabilizes carbocations governs the outcome. The electrophile adds so as to leave the positive charge on the more stable (more substituted) carbon — that is the real, mechanistic meaning of Markovnikov's rule. This single template, electron-rich pi bond meets electron-poor electrophile, underlies hydrohalogenation, acid-catalyzed hydration, halogenation, halohydrin formation, and more; learn it once and a dozen reactions fall into place.
Adding HBr to propene (CH3-CH=CH2): the pi electrons grab H+ to make the more stable secondary carbocation CH3-CH(+)-CH3, then Br- bonds to it, giving 2-bromopropane (CH3-CHBr-CH3) as the major product.
Step 1 makes the more stable carbocation; step 2 traps it with the nucleophile.
The carbocation intermediate can rearrange to a more stable one before the nucleophile arrives, sometimes giving an unexpected product — a tell-tale sign that a free carbocation, not a concerted process, is involved.