The bitrate or data rate dictates the amount of data in kilobytes per second that the video plays back at. Some codecs have a constant bitrate some can have a variable bitrate which allows more data in high action high contrast or high colour scenes and less during static plain or pitch black scenes. Basically when you change this number you are pretty much affecting the final filesize and overall visual quality. The higher the number the better quality but bigger filesize and vice versa. In FCP when you export with a Quicktime conversion some of the codecs will allow you to adjust the target bitrate or data rate for more compression fine tuning. Keep note that higher resolutions require a higher bitrate as there are more pixels and effectively more data needed to transfer. Hope this helps.

 Come on man what do you take me for? I know what bitrate means!

When I go to render settings in Vegas I have the option of 'keyframe every... frames'. I assumed this was the same as your stated 'key frame rate' but I guess not.

 "what does 'key frame rate' actually mean?" Sounds like you asked me what it means

Yep it sounds like its the same thing. Key frame rate is the rate at which the key frames are placed, effectively how many frames apart they are. So keyframe every 250 frames is the same as a key frame rate of 250.

 Ah ok. What effect does this have then? Cheers.

 with keyframes every 250 frames it just checks that the frame reordering hasn't mucked anything up every 250 frames or 10 sec @ 25p. It gives slightly smaller file sizes than the default of every 24 frames which checks more than once a second.

 Ah right sound. Thanks for that. I shall definatly be changing my every 3 frames to something more like your 250!

 that means 50 half frames per second, when it's played back on a computer you will have to convert it to 25 full frames

 No sorry, I wish! That is just key frames for frame reordering.

 50 fps straight off the camera converted to 25 fps giving 50% speed

 so it's still pretty smooth once you slow it down. 1/2 speed would be 25 right which is pretty much my camera's standard framerate .