What is the meaning of Grotto?

A small cave.

An artificial cavern-like retreat.

A Marian shrine, usually built in a cavern-like structure.

A local organization of cavers that typically organizes trips to caves and provides information and training for caving; a caving club.

A secretive name for a local group of underground Satanists.

A garden or roadside shrine with a small cave containing a statue of the Blessed Virgin Mary (Can we verify this sense?)

Source: wiktionary.org