CBR and VBR - the real meaning

CBR stands for Constant Bit rate. What this actually means is that the bitrate doesn't vary much over time. Very few codecs can guarantee exact constant bitrates, so there is almost always some variation.

VBR stands for Variable Bit rate. This means that the bitrate is allowed to vary to a larger amount to maintain better quality over the entire clip.

CBR is useful when streaming video over a severely bandwidth-constrained channels that can maintain fixed speeds, like dialup internet, ISDN, Broadcast television, or Cable or Satellite TV Channels.

VBR is useful when providing video over connections that work better with overall lower speed and occasional spikes of high speed, like broadband internet.

CBR tends to waste space on easy-to-compress sections of video and lose quality on hard-to-compress sections.

VBR tends to produce better quality at any given average data rate.

I generally recommend VBR unless CBR is required for technical reasons.

2 Pass Encoding - the real meaning

2 Pass encoding compresses the video twice - the first time solely to determine how large each frame tends to be, the second time to actually compress with optimized frame size and/or quality.

This allows the encoding program to correct for variations in compressibility and maintain a more even Data Rate. 

When combined with Variable Bit Rate, the encoding program can use the information to improve quality in difficult sections of the video.

Display Aspect Ratio - The Real Meaning

This is the relative height/width that the final video should be played back at.

Non-widescreen Standard Definition TV is 4:3, Widescreen and High Definition TV are 16:9.

Movies vary quite a bit, from very old movies in 4:3, older movies in the Academy standard of 11:8, others in 1.85:1 or 2.40:1, and there were a wide variety of other widescreen movie aspect ratios.

Same as Source means the program will read the original video and use that aspect ratio.

You will sometimes see 1:1 or Square pixels listed. This means that the aspect ratio is proportional to the number of pixels in each direction. this is generally correct for video to be watched on a modern computer (you should resize the video to be proportional to the video's actual aspect ratio if needed.)