a network former
If a glass is a frozen 3D net of atoms, then a network former is the oxide that actually knits the net, and a network modifier is the ingredient you add to cut some threads and make it easier to work. Silica (SiO2) is the classic former: its SiO4 tetrahedra link corner-to-corner in every direction into a continuous mesh, and that mesh is the glass. Boric oxide (B2O3) and phosphorus oxide (P2O5) are formers too.
Pure silica glass is superb but stubborn: because the network is so complete, you must heat it above about 1700 degrees C before it flows enough to shape. So glassmakers add network modifiers, oxides like soda (Na2O) and lime (CaO). Their metal ions do not join the network; instead they break some of the Si-O-Si bridges, leaving dangling non-bridging oxygens. Each broken bridge is a snipped thread, so the net loosens, and the glass softens and flows at a much lower temperature, around 1000 degrees C. Ordinary window glass, called soda-lime glass, is roughly 70 percent silica former plus soda and lime modifiers, which is why it is cheap to make.
The trade-off is that more modifier means easier working but a weaker, less chemically durable, more heat-sensitive glass; too much soda and the glass would even dissolve in water. This is also why the two classes have a middle group, intermediates like alumina, which can play either role. Do not picture the modifier ions as filling holes decoratively; they are actively cutting the load-bearing network, and that is precisely how they lower the melting point.
Pure silica needs about 1700 degrees C to work; add soda and lime modifiers to snip Si-O-Si bridges and soda-lime window glass softens near 1000 degrees C instead.
Formers build the load-bearing net; modifiers cut threads to lower the working temperature, at a cost in durability.
Modifiers do not gently fill gaps; they break bridging Si-O bonds. That is why more soda gives easier working but a weaker, less durable glass, one that can even leach or dissolve.