When an olefin is treated with aqueous acid, #H^+# adds as an electrophile. And so the most stable resultant carbocation is the one with most substitution. Propylene would give predominately #"2-propanol"# on acidic hydration.
On the other hand, when say #"2-methylpropene"# is treated with borane in THF, the hydrogen adds as an hydride:
#H_2C=C(CH_3)CH_3 +B""^2H_3 rarr
(""^2H_2BCH_2C-C""^2H(CH_3)CH_3)# (I labelled the borane so you could see how the hydrogens add. The #B-H# bond adds across the olefin in a concerted mechanism.)
Here, the electron-deficient boron centre is the electrophile.
Oxidative work up gives the primary alcohol #HO-CH_2C-CH(CH_3)CH_3# and an inorganic borate.