| means "or". true means true? That is, "true" constraint is always satisfied. {P} is just a syntax. It means you can construct lifetime L from point P, and that lifetime is written {P}.

It means that a constraint set is either true, or it is a (smaller) constraint set plus a constraint of the form "Lifetime L1 outlives Lifetime L2 at point P".

The second part means that a lifetime can either be a named lifetime 'some_name, or it can be a location in the control flow graph, written as {P} for P being the location.