KiCad's quantised coordinate system. Default 1.27 mm (50 mil). Wires and pins must land on grid points to connect cleanly. Off-grid endpoints can't reliably join.
How it looks
Drawing
Every wire end and pin lands on a grid dot, 1.27 mm apart by default.
How it works
The schematic page has an invisible grid (1.27 mm, 50 thousandths of an inch, by default).
Wires, pins and labels snap to its points as you draw.
Two ends on the same point are joined; ends a fraction off are not, even if they look touching. That is why off-grid ends are flagged as endpoint_off_grid.
Where it fits
Pin positions inside symbols follow the same grid (pin coordinate / pin offset). A different, much finer grid on the board side is the router's lattice.