For electrons which have been accellerated through a potential of 30 to 500 volts (i.e., their kinetic energy is around 30 to 500 eV), they have a wavelength given by the de Broglie relation which varies from 2.2 Angstroms to 0.5 Angstroms in this case. This fits nicely into the range of distances between atoms in solids and can therefore strongly diffract from them. The recording and analysis of the diffraction pattern can tell us the arrangement of the atoms on the surface. The sharpness of the pattern is related to the extent of order (or conversely, disorder) of the atoms on the surface.
The experiment consists of an electron gun that forms an electron beam with a narrow kinetic energy spread. This beam scatters from the sample surface through a set of charged grids which function to energy select only the electrons which elastically scatter from the surface and are then accelerated to a fluorescent screen. When they strike the screen, they cause the phosphor to glow, revealing a pattern of dots which is the diffraction pattern. Here is a schematic representation of the process.
This is a photograph of a LEED image taken with an incident electron beam energy of 65 volts on a platinum surface. The arrangement of the spots is interpreted to provide information about the ordered arrangement of atoms on the surface and the distances between the spots gives information on the distance between the atoms. The sharpness of the spots gives insight on how well ordered the surface atoms are arranged.