The First Letter Key is simply the first letter of every word (usually in the form of a card or booklet).
The text
I am the vine;
you are the branches
made into a First Letter Key would become
Iatv; yatb.
This simple device is a great help in memorizing the Scripture. It provides a useful “prompting” step between not knowing the material (having to read it off the page) and having it completely memorized.