Bitrate: specifies the bitrate per channel the AAC encoder uses, in bps. Although there are some fixed settings you can cycle through with the up-down-buttons, any bitrate can be typed into the field.

Bandwidth: specifies the bandwidth the encoder uses of the audio input file, in Hz. Higher frequencies will be cut off.

Mpeg Version: lets you choose the MPEG version the output AAC file will be. Either 2 or 4.
Note: libfaac currently doesn't support MPEG2 and the LTP object type at the same time.

AAC Object Type: lets you choose the object type that will be used for the output file.

allow Mid/Side: allows the AAC encoder to use Mid/Side encoding. Stereo signals are separated in a mid (left minus right) channel, and a side (left plus right) channel. Should only be used when there is low stereo separation.

use TNS: tells the AAC encoder to use Temporal Noise Shaping.

See also:
Working with Movkit DVD Ripper Pro