Although she has never seen a Moor (open, overgrown land) or the Sea, she claims that she knows how they look like. Likewise, in the second stanza, she says that although she "never spoke with God...
In one of her poems, "I Never Saw a Moor", it most likely relates to the North Yorkshire Moors where heather plants grow. But, in another poem, "Wild Nights" it means to anchor to or to hold in place...