Hi sorry i missed this one - not too hard for "biology"
If you look at a protein crystal (or any crystal really) in an x-ray beam, it scatters lots of spots (diffraction reflections). If you look at a picture of crystalline diffraction, at larger angles from the center of the x-ray beam, the reflections get weaker and weaker and basically just stop, if the wavelength is short enough (in all crystallography labs its plenty short - from 1.5 to 0.9 Angstroms).
The resolution is marked by the angle of scattering to the last measurable spot (aesthetics can vary here, but there is little variance from personal judgement here). Once you have the angle of scattering, you can calculate the resolution from the Bragg scattering equation:
lambda = 2d sin(theta)
which is rearranged to solve for 'd'
d = lambda / 2 * sin (theta)
where lambda is the wavelength of the incident radiation and theta is the scattering angle.
d = the apparent width of the 'slit' which caused this highest resolution reflection is called the 'resolution' of the X-ray scattering experiment.
There is one part that's a bit tricky as in the typical diagram, depicting Bragg reflection/scattering, you would take the incident x-ray beam as first beam and the scattered angle as the beam of the high resolution scattering, which is more easily measured as 180 - 2* theta.