Each grain in a solid, particularly polycrystalline solid, has sources of dislocations. S1 and S2 are simply dislocation sources in two grains. Grain 1 is 'more favourably oriented' to the tensile stress in the example, so it achieves the critical shear stress to let dislocations glide along the slip planes before Grain 2, which does eventually experience dislocation movement.
With regard to dislocations, see - the previous page - http://aluminium.matter.org.uk/conte...eid=1000298442
Tony, What text are you using?