Hypothesis: Let $F$ be a finitely presented group s.t.
$$ F = \left\langle S \mid R \right\rangle $$
Let $X$ be a $4$-manifold.
Question: I've seen it asserted that we can represent each relation in $R$ by a loop in $X$. But in what sense can we represent relations in $R$ by loops in $X$? What precisely does this mean?