the critical resolved shear stress
Dislocations glide when they are pushed sideways, by a shear stress, the kind that slides one layer past another, not the kind that pulls layers apart. The critical resolved shear stress (CRSS) is the threshold: the exact amount of shear stress, felt on the slip plane and along the slip direction, needed to start dislocations moving. Below it the crystal only stretches elastically; reach it and the crystal begins to yield.
The word resolved matters. When you pull a bar in tension, the slip plane usually sits at an angle to the pull, so only a fraction of your stress acts as shear along the slip direction; you have to resolve (project) the applied stress onto that plane and direction. CRSS is the value of that resolved shear component at the moment slip starts. It is a property of the material and its condition: a fixed number for a given metal at a given temperature and purity, independent of how the sample happens to be oriented. Pure single-crystal metals have a very low CRSS (copper is under 1 MPa), and every strengthening mechanism in this field works by raising it.
CRSS drops as temperature rises (atoms jiggle more, helping dislocations over obstacles) and climbs steeply as you add impurities, shrink grains, or tangle in more dislocations. It is the microscopic quantity that sits behind the macroscopic yield strength you measure in a tensile test.
For pure FCC copper the CRSS is roughly 0.5 to 1 MPa, but the tensile bar yields at a higher stress because the applied pull must be several times the CRSS before its resolved shear component reaches that threshold.
CRSS is the true trigger for slip; the yield strength you read off a test is that trigger seen through the sample's orientation.
CRSS is a shear stress on a specific plane and direction, not the tensile yield strength; they are related through orientation (Schmid's law), and confusing the two gives numbers that are off by a factor of two or more.