In printed circuit board design, a net refers to an electrical connection between two components (especially in schematics). They’re not individual wires: in fact nets can branch off into different sections to connect multiple components.
For convenience, nets have a specific name (either assigned manually or automatically by the CAD software) instead of connecting things by explicit wires. Also by convention, it’s a good idea to name power nets after the voltage level and type of power (i.e., +12V_DC) so there’s no confusion.