"Mother Lode"

Hi. Does anyone know what the phrase "Mother Lode" stands for and means? I have never heard of it before and just saw it in a magazine article.

Thanks

Merriam-Webster:

Main Entry: mother lode
Function: noun
1 : the principal vein or lode of a region
2 : a principal source or supply

Main Entry: lode
Pronunciation: 'lOd
Function: noun
Etymology: Middle English, from Old English lAd course, support; akin to Old English lIthan to go -- more at LEAD
1 dialect English : WATERWAY
2 : an ore deposit
3 : something that resembles a lode : an abundant store