What is bitrate?

Bitrate means the number of bit which go through the stream per 1 sec. Stream means the MPEG file.
Generally, compression is described by bitrate in MPEG. The higher bitrate, the higher quality and lower compression, larger file size.
If the bitrate is same at any part on a single stream, it is called as CBR (Constant Bit Rate). In VBR(Variable Bit Rate), the bitrate can be different depends on the part of stream.