MBAFF, or Macroblock-Adaptive Frame/Field Coding, is a video encoding feature of MPEG-4 AVC that allows a single frame to be encoded partly progressive and partly interlaced. Maintaining the quality of interlaced video can be a challenge in video encoding because of the larger spaces between horizontal lines in the same field. MBAFF allows an AVC encoder to examine each block in a frame to look for similarities between interlaced fields. When there is no motion the fields will tend to be very similar, resulting in better quality if you encode the block as progressive video. For blocks where there is motion from one field to another the quality is more likely to suffer if encoded progressive, so these blocks can remain interlaced.
In addition to quality improvements, progressive frames require fewer bits making them more compressible for the same quality as interlaced frames. This also applies to individual macroblocs, meaning that by using MBAFF you may greatly improve quality for a given bitrate. Since all the decisions required for MBAFF are made by the encoder it can slow encoding speeds greatly. Being a relatively new technology it's also not supported well among AVC playback software. Unlike many of AVC's advance features, though, MBAFF doesn't affect decoder speed.
