$\text{Ph}-{\text{CH}}_2-\text{CH}={\text{CH}}_2\stackrel{{\text{H}}^{+}}{\to }\text{Ph}-{\text{CH}}_2-\stackrel{+}{\text{C}}\text{H}-{\text{CH}}_2\to \text{Ph}-\stackrel{+}{\text{C}}\text{H}-{\text{CH}}_2-{\text{CH}}_3\begin{array}{c}{\text{H}}_2\text{O}\\ \to \\ -{\text{H}}^{+}\end{array}$

In the first step, a proton from the acid is added to the compound and the respective carbocation is formed. In the next step, a hydride shift occurs to give a more stable benzylic carbocation which is extremely stable.

In the last step, an OH^- is added to give benzylic alcohol