Think of it this way: you've probably heard about the million monkeys on a million typewriters will eventually produce Shakespeare? Ultimately something deeply meaningful like that is just a sequence of symbols and you generate enough randomo sequences of symbols, one or more of them will be literature.

A Boltzman Brain is basically that but for a quantum system. In the far future, it's suspected the Universe will undergo heat death and eventaully reach thermodynamic equilibrium but that doesn't mean nothing happens on a quantum level. A chaotic system as otherwise thermodynamic equilibrium is still capable of having temporary, localized order without violating entropy.

So over any sufficiently long period of time (we're talking 10^100000+ years here), you'll get all sorts of interesting arranagements of matter. One of these is a localized reality where it or being in it are capable of sentience.. They won't have any conception of the Universe outside of this but, for a time, they will be able to contemplate their reality.

Think of it another way: take a bucket filled with random elements, mix it and tip it on the floor. Do this enough times and one of those bucket pours will be sentient life.