Recall that when a conductor is exposed to an electric field, the free electrons in the metal move at drift velocity. If we expose this conductor to a perpendicular external magnetic field, each electron will experience a force: $$ \begin{equation}\begin{aligned} F&=BQv\sin{\theta}\\ F&=Be\nu\\ \end{aligned}\end{equation} $$ since $e$ is the charge on an electron and $\nu$ is the drift velocity of an electron. The angle $\theta=90\degree$ due to the magnetic field being perpendicular.
The result of this is that a potential difference will be set up as the electrons (and thus negative charge) will shift to one side of the conductor. This potential difference is referred to as the Hall voltage.
The Hall probe
This device uses the Hall effect in order to measure magnetic flux density. A current, $I$ is passed through a semiconductor strip and the strip exposed at right angles to the magnetic field being measured.
Image Credits: HyperPhysics
The Hall voltage, $V_H$ is then measured and the magnetic flux density calculated using the formula: $$ \begin{equation}\begin{aligned} B=\frac{V_H\times net}{I}\\ \end{aligned}\end{equation} $$
The value of $net$ will be provided by the probe manufacturer. This scalar is determined through calibration of the probe using known magnetic fields and corresponding Hall voltages.