When alkenes undergo electrophilic addition the mechanism establishes that not all products are formed in equal amounts and this is explained through Markovnikov’s Rule;
In an addition reaction of a protic acid HX to an alkene or alkyne, the hydrogen atom of HX becomes bonded to the carbon atom that had the greatest number of hydrogen atoms in the starting alkene or alkyne.
What this rule says is that when a positive atom interacts with the electron cloud of the alkene the new bond will form on the Carbon that has the highest number of hydrogens (this can also be explained as the Carbon that has the least number of alkyl groups attached to it).
This is due to the fact that alkyl chains stabilise positive charges through a process called hyperconjugation. This ends up having a similar effect to the positive inductive effect, in which electrons are pushed towards the positive carbon, stabilising the charge and thus making the intermediate more stable and more likely to react.
What this means is that a tertiary carbon would be more stable than a secondary carbon which is more stable than a primary carbon. It must also be noted that there is no difference in stabilisation when the alkyl group is short or long. A methyl would offer the same stabilisation as an ethyl or a propyl.