Shield Damage is the Smash Bros. equivalent of Chip Damage. Unlike chip damage, which deald reduced damage when an opponent blocks, Shield Damage doesn't even deal damage. Instead, it makes your shield smaller and smaller with every hit. When it becomes as small as possibly, a character is sent into automatic Hit Stun. It remains basically unchanged in Super Smash Bros. Battle Royale.

In-Depth Explanation

Jigglypuff Shield Break