When a particle decays, it must obey the laws of physics, which include certain conservation rules.  For example, energy must be conserved - the total energy of all decay products must be equal to the total energy of the original particle.  Electric charge must also be conserved - the sum of electric charges on all decay products must be equal to the electric charge of the original particle.  And so on.  In general, this means a particle decays to two or more particles whose total mass is less than that of the original particle.  In addition, each interaction has its own set of conservation rules, so how a particle decays depends on what it can do and still obey the rules.  It might be able to do more than one combination of things.

The B meson would be stable if the b-quark and companion antiquark didn't have weak charge.  Because they do, and because the meson is heavier than many other mesons, there are many ways in which it can decay (for a link to a summary report, see our links page).  All of these ways involve the b-quark transforming itself into another quark, which could be a t, c, or u quark.  If it's a t, the t must then be transformed again, to a quark that's lighter than b, because t is more massive than b and couldn't be there in the end and still obey the laws (this involves upper college-level physics, so we won't belabor it here). Sometimes the companion antiquark also gets transformed.  In any case, many of these transformations can be detected experimentally, and we can measure the weak charge associated with them.