VBR stands for Variable Bit Rate.
It is a term often used with video encoders, to indicate that the encoder works in a variable bit rate, generating different amount of bits per second based mainly on the content. The end result over a long period will be on average the bitrate we are aiming for, but there are no guarantees that short periods of time will adhere to that bitrate.
VBR encoding schemes aren’t popular with real time media since they don’t adhere to bandwidth constrains and may cause congestion and latency. VBR is more popular with screen sharing encoding (where content is mostly static in nature), recording and streaming of pre-recorded content where latency isn’t as hard a requirement as in live streaming.
In WebRTC, VBC is used by default for screen sharing content (unless stated otherwise).
The other popular encoding scheme is CBR.