ROOT is a program package and data format used by CERN (the European particle-physics laboratory where Tim Berners-Lee invented the Web) in conjunction with its Open Data initiative to make much of its raw data accessible to the public for whatever analysis outside people might wish to do. ROOT is a set of functions to analyze data, accessible by various GUI and command-line methods as well as being usable as library routines in programs others may wish to develop. CERN data has been released in varying formats depending on the nature of the data, but much of it is in a binary format specific to ROOT.
It appears that ROOT files begin with the four bytes (hex)
72 6F 6F 74, which spell "root" in ASCII.