Here is the context in which I heard it. If $G$ is a reductive group over a non-archimedean local field $F$, then let $K$ be the hyperspecial maximal compact subgroup $G(\mathcal{O})$. For example, if $G=SO(n)$, then it's a split $SO(n)$, and there is a natural choice of $SO(n)(\mathcal{O})$. What exactly does the last sentence mean? The only context I am aware of is in the theory of Lie groups (https://en.wikipedia.org/wiki/Indefinite_orthogonal_group#Split_orthogonal_group).
Edit: There is an answer here: https://mathoverflow.net/questions/258678/what-is-a-split-son