The count of footprint pads that have a net assigned. The rest are NC pins or mechanical (mounting holes).
How it looks
Real pad positions and nets from CORE3588, tracks hidden
The USB-C corner of CORE3588. Amber: pads joined to a real net. Dark grey: pins marked no-connect. Black holes: mounting or peg holes with no net at all.
1A plastic-peg hole of the USB-C socket: no copper, no net. One of the 8 pads on the board that are not counted.
2A USB-C pin the design does not use (SBU2). It sits alone on a no-connect net: dark grey here. Its twin SBU1 is the other dark grey pad above.
3The other USB-C pins: amber, each on a real net (power, ground, the data pair, the CC lines).
How it works
Counted from our board file
Every pad in a board belongs to a net or it does not. KiCad counts the ones that do. On CORE3588: 1065 pads, 1057 with a net. Of those, 76 sit alone on a no-connect net (an unused chip pin, see NC / no-connect marker), so 981 pads carry a real connection a router must make.
Where it fits
The pads with a net are the router's raw material: the netlist groups them into nets, and the ratsnest shows which still need joining.
Pads without one are mechanical (mounting holes, non-plated holes) and are only obstacles to route around.