Prism direction "in" (base in) and "out" (base out) refer to the orientation of the prism in the eyeglass lenses, used to correct horizontal eye alignment issues:

Base In: The thicker part of the prism is towards the nose. This is used to treat exotropia (when the eyes tend to drift outward) by redirecting light inward, encouraging the eyes to converge.
Base Out: The thicker part of the prism is towards the temples (away from the nose). This is used to correct esotropia (when the eyes drift inward) by shifting light outward, helping the eyes diverge.

The prism direction is tailored to each person's specific misalignment to help the eyes work together more effectively.

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