The reaction between 3-methylbutan-2-ol and $\mathrm{HBr}$ is explained below.
In the first step, $-\mathrm{OH}$ group is protonated. In the second step, a molecule of water is lost to form secondary carbocation. Third step is the rearrangement of less stable secondary carbocation to more stable tertiary carbocation through $1,2-$hydride ion shift. Final step is the nucleophilic attack of bromide ion on tertiary carbocation to form 2-bromo-2-methylbutane as shown below.