selection rules
Out of all the reactions you could imagine writing down, most never happen. A heavy particle has, in principle, countless lighter combinations it could decay into — yet it picks only a few, ignoring the rest entirely. Selection rules are the practical shorthand for which processes nature permits and which it forbids. They are the gatekeepers that say 'this decay is allowed, that one is not,' before you even calculate how likely the allowed ones are.
Selection rules are simply the conservation laws and symmetries applied as a checklist. Before a process can happen, the totals of all the conserved quantities must match on both sides: energy, momentum, angular momentum, electric charge, baryon number, lepton number, and — for the relevant forces — flavor quantum numbers and the discrete parities. If any one of these does not balance, the reaction is strictly forbidden, no matter how much energy you supply. If everything balances, the reaction is allowed (though it may still be rare for other reasons). For example, a process that would change total electric charge is absolutely forbidden; a process that changes strangeness is forbidden for the strong force but allowed for the weak force.
Selection rules are how the abstract symmetry principles cash out into everyday predictions in the lab. They explain why a given particle has the specific decay products it does and not others, and why some decays are fast (all rules satisfied by the strong force) while others are slow (they require the weak force because some flavor must change). They also come in degrees: a hard-and-fast rule, like charge conservation, can never be broken, while an 'approximate' selection rule tied to an approximate symmetry can be violated occasionally, producing rare but real decays. Spotting a process that breaks a supposedly exact selection rule would be a discovery of new physics.
A neutral pion (no strange quarks, strangeness 0) decays into two photons in a tiny fraction of a second, because the fast strong and electromagnetic forces can do it. A neutral kaon (strangeness 1) lives billions of times longer, because its decay must change strangeness — forbidden to the strong force, so it has to wait for the slow weak force.
Which force a decay needs sets its speed, and selection rules decide which force can do it.
A selection rule says whether a process is allowed, not how often it happens — an allowed decay can still be rare for dynamical reasons. And 'forbidden' is only as firm as the symmetry behind it: approximate symmetries give rules that bend, exact ones give rules that hold absolutely.