I’m trying to find out! b-jets are flows of particles that are “initiated” by bottom quarks… and even that definition is a bit ambiguous, so we need to be very careful about what we mean. The bottom quark has two properties that are important:
First, the structure of the theory means that they are very long-lived for quarks, and this means that we can identify (“tag”) those jets by looking for evidence of them having flown a little distance in the detector, rather than decaying immediately.
And second, they are heavy compared to other quarks (other than the top quark, which is a totally different kettle of fish). The quark mass controls how it emits strong-force radiation in the form of other quarks and “gluon” particles — the pattern of energy distribution in the jet is then expected to be different from that in jets initiated by light quarks or gluons. The theory starts to break down at some level of detail, so we currently don’t know exactly how it works: that’s why I’m making measurements so we know which theories to throw away and which to keep!