Letting the code speak, he means tracing a ray from a player's position down and finding the first intersection with map geometry.
The reason why you need that value is that probably, TR_TraceRayEx takes an angles structure as its direction parameter when used this way,
ie. pitch/yaw/roll (in that order). Pitch is rotation around the Y-axis, yaw around the Z-axis, roll around the X-axis according to
Valve's dev wiki (all in degrees). What is being rotated is the normal vector in X-direction ( ie. the vector 1.0 / 0.0 / 0.0 ) (that is the forward output of the AngleVectors function). So what you need is 90 degrees clockwise or counter-clockwise around the Y axis.
Disclaimer: I have never used TR_TraceRayEx so I may be wrong.
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