Bitrate is how many bits per second a camera's video stream consumes — the single number that most directly drives both the network load and the storage bill. A higher bitrate means more detail and smoother motion but bigger files; a lower bitrate saves space and bandwidth at the cost of quality. It is the output of the encoder's settings (codec, resolution, frame rate, compression) and the input to almost every capacity calculation in a surveillance design.
The storage maths runs straight off it: storage equals bitrate times time times the number of cameras times the recording factor. A useful rule of thumb is that 1 Mbps of continuous video is about 10.8 GB per day, so a single 4 Mbps camera recording continuously consumes roughly 43 GB a day, and a hundred of them about 4.3 TB a day. Getting the bitrate estimate right is therefore the foundation of sizing storage and network — everything else multiplies it.
Two subtleties are common pitfalls. First, constant vs variable bitrate (CBR vs VBR): VBR spends bits only when the scene is busy, so its average can be far below its peak, but you must size storage for realistic averages and the network for peaks, not confuse the two. Second, a "smart codec" can advertise a low average that balloons during motion (the exact times you most need quality), so sizing on the quiet-scene figure under-provisions the system. Plan storage on measured, scene-realistic bitrates with headroom, not on the datasheet's best-case number.
