Average Bit Rate
Enable this option to indicate the long-term desired average data rate (in kilobits per second). A higher value results in better image quality but bigger file size. Note that this is not a hard limit; the bit rate may peak above this. If disabled, the encoder does not set a limit.
Maximum Key Frame Interval
Enable this option to tell the encoder to generate at least one key frame for every number of frames specified. A smaller value gives a video that is easier to navigate during playback (due to the increased number of key frames), but the file size may increase in this case. If disabled, the encoder should choose where to place all key frames. It can generate key frames more frequently if this would result in more efficient compression.
Here you specify the bitrate the video is encoded at. The higher the value, the better the quality of the picture and the fewer artifacts (macro blocks and blurring). You should experiment with this. Please note that higher values result in a larger file size.
The mean bit rate, and the main value that affects quality. Maximum bitrate should generally exceed this, but no set margin is required; the Average and Maximum bitrate can safely be the same.
This is the absolute maximum bitrate. It does not dramtically affect quality but some formats have set values that must not be exceeded (e.g., 10 megabits for DVD video). If your maximum bitrate is set too low, then you will get macro block effects, in which case you should increase BOTH values.