RIFF stands for Resource Interchange File Format and is a container format for many files including WebP.
RIFF provides a universal recognized header format that can be followed by a chunk of data in the appropriate format. The header identifies the type of data in the file and its size. It has very low overhead consisting of 12 bytes. The first 4 bytes are the letters RIFF followed by 4 bytes identifying the file size and then 4 bytes identifying the format of the data. For WebP, as an example, those 4 letters would be used.
Some formats can add additional header data to provide specialized features unique to the format such as a feature description, a ICCP color profile, animation control data, and metadata such as Exif and/or XMP.