Distance between a point and a given line segment

I’m working on line simplification algorithms,I choose ramer douglas peucker algorithm, I found this python implementation:https://github.com/fhirschmann/rdp but I didn’t understand how they calculate the distance between a point and a line segment in (pldist function) can anyone give me more details and explain to me how this function do.

def pldist(point, start, end):
“””
Calculates the distance from point to the line given
by the points start and end.

:param point: a point
:type point: numpy array
:param start: a point of the line
:type start: numpy array
:param end: another point of the line
:type end: numpy array
"""
if np.all(np.equal(start, end)):
    return np.linalg.norm(point - start)

return np.divide(
        np.abs(np.linalg.norm(np.cross(end - start, start - point))),
        np.linalg.norm(end - start))