slip
When you bend a metal paperclip and it stays bent, something inside it has permanently moved. That something is slip: whole sheets of atoms sliding over one another along particular planes in the crystal, like a deck of cards shearing sideways. Slip is the basic way metals deform plastically (permanently) instead of just stretching elastically and springing back.
Slip does not happen by a whole plane of atoms breaking free and sliding at once, which would take an enormous stress. Instead it happens one line of atoms at a time, by moving a defect called a dislocation through the crystal. Picture moving a heavy rug across a floor: dragging the whole rug is hard, but kicking a small wrinkle and walking that wrinkle from one end to the other moves the rug with little effort. A dislocation is that wrinkle in the lattice, and each time one sweeps across a slip plane the crystal shifts by one atomic step.
This picture explains one of the great puzzles of materials science: real metals yield at stresses 10 to 100 times lower than a perfect crystal should need. They are weak because dislocations let them slip easily. Turned around, every trick for making a metal stronger is really a trick for making slip harder, jamming dislocations so they cannot glide. That is the theme of this whole field.
A single crystal of zinc pulled in tension shows visible steps on its polished surface where slip planes have sheared past one another; these slip lines (or slip bands) are the atomic sliding made large enough to see.
Slip bands on a deformed surface are the footprints of millions of dislocations gliding on the same family of planes.
Slip is plastic (permanent) deformation, not elastic; do not confuse it with the reversible stretching of bonds that gives Young's modulus. Slip is also not fracture: the metal flows and stays in one piece.