m_pitch and m_yaw are two variables that describe your aiming geometry, m_pitch for vertical and m_yaw for horizontal motion.
when they are both the same, if you move your mouse in a circle, your crosshair moves in a circle.
lowering one "squishes" the circle your crosshair travels in when you move your mouse in a circle..
a lot of players lower m_pitch so that when they move their mouse in a circle, the crosshair movement looks more like a jelly bean laying on its side. or a real bean.. they aren't shaped so differently =)
they do this to lessen horizontal movement of the crosshair, so the crosshair stays closer to head-height on the character models.
once you are used to it it makes it easier to keep from shooting at peoples shoes instead of their face, but the tradeoff is that it's slower to aim up or down at proners or very tall players.