Variable bitrate

Constant bitrate holds the same rate throughout. That wastes data on a static talking head and starves a shot full of motion, rain or confetti. VBR moves the budget to where the picture needs it, so at the same file size the quality holds up more evenly across the video. Two-pass VBR analyses the whole file before encoding it and allocates far better than one pass, at the cost of a longer export. Constant bitrate still has its job: live streaming, where the network wants a predictable rate and there is no second pass to run.