The MD3 Export Options window is displayed when you save a model in Quake MD3 format. See Quake MD3 for more details about player models and animation.cfg.
The Save as a Quake 3 Player checkbox allows you indicate if you want to export as a Quake 3 composite player model (head.md3, upper.md3, and lower.md3 models). If it is grayed out, see the list of requirements for export on the MD3 page.
The Save Quake 3 animation.cfg checkbox allows you to indicate if you want to write "animation.cfg" file along side the model. This file contains a list of animations with looping and frames per-second information and can contain other arbitry data from MD3_CFG_* meta data.
Click Ok to save the model with the specified options or click Cancel to ignore any changes.