FEOL and BEOL
Beginner
The two halves of building a chip: first the tiny switches in the silicon, then the layers of wiring on top.
Novice
Front end of line (FEOL) is the first part of wafer processing, which builds the transistors in the silicon surface. Back end of line (BEOL) is the second part, which builds the stack of metal wiring layers above them, separated by insulating layers and joined by vertical plugs called vias.
Expert
FEOL masks are the base layers and the most expensive to change. BEOL masks are the metal and via layers. A late fix that changes only wiring (a metal-only ECO) replaces just those, which is why designers scatter unused spare cells across the chip at layout time: they sit in the FEOL waiting to be wired in.
Explained in After tapeout (Extras).
See also: Metal stack, ECO (engineering change order).