CBR stands for Constant Bit Rate.
It is a term often used with video encoders, to indicate that the encoder works in a constant bit rate, generating the same number of bits per a short period of time (usually 1 second or less).
CBR encoding schemes work well when the network bandwidth is known and limited and is quite common in live video sessions where latency must remain low.
The compression created by CBR encoding schemes are great for sending media live over a network, but not as good when needing to store recordings as they tend to be larger than necessary.
WebRTC uses CBR by default when encoding camera sources.
The other popular encoding scheme is VBR.