Accumulation and ablation zones
A glacier has two distinct zones. Above the equilibrium line, more snow falls than melts, so the glacier gains mass (accumulation). Below the equilibrium line, melt exceeds new snow, so the glacier loses mass (ablation). The equilibrium line marks where accumulation equals ablation. As long as both zones feed material downhill, the glacier remains stable.
Net flow and glacier health
Glacial ice flows downhill under its own weight, compressing snow into dense ice as it moves. If a glacier's accumulation zone shrinks (warming climate, less snow), the glacier retreats as the lower parts ablate faster than they're replenished from above. Most mountain glaciers worldwide are retreating because climate warming is shrinking the accumulation zone.