What does this equation mean?

AlphaFold2 was assessed in CASP14, the blind, independently organized structure-prediction assessment in which competing methods predict structures for proteins whose experimentally solved coordinates are not yet public, and are scored by assessors with no stake in any team’s outcome. Against that standard, AlphaFold2’s median backbone accuracy was 0.96 angstroms of Cα root-mean-square deviation from the experimental structure, against 2.8 angstroms for the next-best method, and its median all-atom accuracy was 1.5 angstroms against 3.5 angstroms for the next-best method [ 9 ] . What that root-mean-square-deviation figure actually measures is worth stating explicitly, because the number is often quoted without the definition it depends on: $\text{RMSD} = \sqrt{\frac{1}{N} \sum_{i=1}^{N} \lVert x_i - \hat{x}_i \rVert^2}$. where $x_i$ is the coordinate of atom i in the experimentally solved structure, $\hat{x}_i$ is the corresponding coordinate in the predicted structure after optimal superposition of the two, and N is the number of atoms compared. This is a statement about the average positional deviation of one predicted, static conformation from one experimentally solved, static conformation. It is not, by construction, a statement about how the molecule moves, how quickly it interconverts between conformations, or what its folding pathway looks like — and that distinction is the entire content of what comes next.