"She found me roots of relish sweet, And honey wild and manna-dew; And sure in language strange she said, 'I love thee true.'
The knight says that the fairy lady found him tasty roots, honey, and manna to eat ("of relish sweet").
"Manna" is the food that the Jewish scriptures say that the Israelites ate when they were wandering around the desert after Moses freed them from slavery in Egypt. It's supposed to be food from heaven, so this word makes the fairy lady seem supernatural, if not actually divine.
Alternatively, the association could be with the slavery from which the Israelites had just been freed. After all, the knight does become enslaved to the beautiful fairy lady. This allusion becomes even more potent when it's associated with the "honey wild" that the fairy lady fed the knight. (The Israelites were trying to find the Promised Land, which would flow with "milk and honey.")
The fairy lady tells the knight that she loves him, but she says it "in language strange."
He doesn't say what language it is, or how he's able to understand her. Maybe he's just hearing what he wants to hear, or maybe her magical influence has enabled him to understand her "language strange."
Stanza 8, Lines 29-32
"She took me to her elfin grot, And there she wept and sigh'd full sore; And there I shut her wild, wild eyes With kisses four.
The fairy lady takes the knight to her "elfin grot." "Elfin" just means having to do with elves, as any Tolkien fans probably figured. And a "grot" is a grotto, or cave.
Once they're back at her fairy cave, she cries and sighs loudly. The knight doesn't say why she's crying, and we never find out – it's left to our imagination.
The knight kisses her weepy eyes four times. (Why "four" kisses? Isn't "three" usually the magic number in fairy tales? )
Again, her eyes are described as "wild," and this time it's repeated twice.