Q:

What exactly does bitrate mean in an video/audio file?

I use ffmpeg to convert videos from one format to another.
Is bitrate the only parameter which decides the output size of a video/audio file?

A:

Yes, bitrate is essentially what will control the file size (for a given playback duration).  It is the number of bits used to represent each second of material.
However, there are some subtleties, e.g. :

a video file encoded at a certain video bitrate probably contains a separate audio stream, with a separately-specified bitrate
most file formats will contain some metadata that won't be counted towards the basic video stream bitrate
sometimes the algorithm will not actually aim to achieve the specified bitrate - for example, using the CRF factor. http://trac.ffmpeg.org/wiki/x264EncodingGuide explains how two-pass would be preferred if targeting a specific file size.

So you may want to do a little experimenting with a particular set of options for a particular file format.

