the equilibrium spacing
Two bonded atoms settle at one particular distance apart, not touching, not far, but a specific gap that they return to whenever disturbed. That preferred distance is the equilibrium spacing, usually written r0. It is simply the bottom of the energy valley, or equivalently the distance at which the net interatomic force is exactly zero: attraction and repulsion in perfect balance.
For most solids r0 is a couple of angstroms (a few tenths of a nanometre): about 1.5 angstrom for a carbon-carbon single bond, about 2.8 angstrom between neighbouring sodium and chlorine centres in salt. Roughly, the equilibrium spacing is the sum of the two atoms' radii, which is exactly why we assign atoms sizes at all. Push the atoms closer than r0 and they push back (repulsion wins); pull them past r0 and they pull back (attraction wins); leave them alone and they sit at r0.
The equilibrium spacing is what fixes the size of a unit cell and therefore the density of a crystal; the lattice parameters are just equilibrium spacings stacked up in three dimensions. It also shifts with conditions: heating widens the average spacing (thermal expansion, because the valley is lopsided), and squeezing under pressure shortens it. So the bond length you measure by diffraction is really this equilibrium point, read off the atomic arrangement directly.
In rock salt, each Na^+ and Cl^- centre sit about 2.8 angstrom apart, close to the sum of the sodium ion radius (about 1.0 angstrom) and the chloride ion radius (about 1.8 angstrom). That single spacing, repeated through the crystal, sets the unit-cell edge and hence salt's density.
The resting bond length is the sum of atomic radii and the seed of the unit cell.
The equilibrium spacing is a slightly idealised average: real atoms are always vibrating, so at any instant the distance fluctuates around r0. What diffraction reports is the time-averaged spacing, which is why it drifts (slightly outward) as temperature rises.