Toughened or tempered glass is heated until it softens, then its surfaces are cooled rapidly. The outside solidifies first while the interior is still hot. As the center cools later, the resulting stress pattern leaves the surfaces in compression.
Cooling creates stored stress
A crack at the surface must overcome that compression before it can grow, which increases resistance to bending compared with ordinary annealed glass. When the stored stress is released by a serious break, the pane tends to fragment into many small pieces rather than a few long sharp shards.
Strong does not mean unbreakable
Edges and deep scratches remain vulnerable, and tempered glass can still fail. It cannot normally be cut or drilled after tempering without disrupting the stress pattern. Laminated glass solves a different safety problem: its interlayer helps hold fragments together after a pane breaks.
Related: how laminated glass holds fragments together.