The Legend Gauge (Legenddo Geji) is a zone where Legend Materials for Legend Monsters go.
When cards are used for a Legend Summon, they are placed into the Legend Gauge where they are stacked in a way all players can easily tell how many cards are in it.
The Legend Gauge is a zone located just off to the side of the board where Legend Material monsters are stacked. Cards in this zone are not considered on the field, so if an effect would destroy cards on the field, these cards cannot be targeted or affected.
Legend Summoning is not the only way to put cards into the Legend Gauge. Card Effects can also increase the Legend Gauge, but these are either limited in their activation or specific in the types of cards they can affect. The card effect will say where to get the cards from.