Definition
Brotli compresses a stream with a large built-in dictionary of common sequences, plus matches against data it has already compressed. A context model then encodes those matches. The file is often smaller than the same data in Gzip, at a similar decompression cost.
Relevance
Brotli cuts download size for a WebGL game when your server can send it. Decompression cost stays close to Gzip, so the gain is a smaller transfer for browsers that accept the format.
Example
You want to publish a WebGL game with a smaller download. Gzip is a well known format that all browsers support. Switching Compression Format to Brotli shrinks it further for browsers that accept Brotli.