What does this equation mean?

It is worth dwelling on how unusual this situation is for an empirical science. In most fields, a persistent discrepancy between two measurements of the same quantity is resolved by finding the weaker method and improving it. Here, both methods have already withstood roughly two decades of adversarial scrutiny. The distance ladder has been rebuilt with three independent stellar calibrators — classical Cepheids, the tip of the red-giant branch, and Mira variables — each anchored by a different absolute-distance technique, and each produces a local $H_0$ well above Planck’s inferred value, though not always identical to SH0ES’s own number. The CMB-based inference has likewise been cross-checked against an entirely separate CMB experiment, the Atacama Cosmology Telescope, and against the South Pole Telescope, both of which recover parameters consistent with Planck’s. This is why the tension is treated as a serious anomaly rather than a rounding error to be absorbed by better bookkeeping: the redundancy on both sides of the disagreement has already been built, and it did not make the gap close.