settle can only understand wiki-style links, such as [[Neurons]], or
[[Language learning]], which would redirect to a note called Neurons or
Language leraning, respectively. Case-in-point, a wiki-style link is any text
between two matching [[ and ]].
Such links may be embedded anywhere inside a note.
If we're considering some note X, then its forward links are the set of every note that X links to. Meanwhile, its backlinks are the set of every note that links to X.
More concretely, let's say we have two notes, note X that links to note Y. Therefore, X has Y as a forward link, Y has X as a backlink.
Since only the bare minimum of metadata is stored, backlinks aren't actually kept in the database: they can be deduced, or, rather, computed, from forward links. Also note that, of course, backlinks aren't stored anywhere within the notes' contents.