Oven spring is the early increase in a loaf's volume as gases expand and steam forms in hot dough, before the crumb and crust set. Dough structure, fermentation and oven conditions all influence it.
A moist surface and well-placed scores allow expansion where intended. More oven spring is not always better: an underproofed loaf may burst unevenly, while an overproofed loaf may have little strength left to rise.
Reference: Further reading.