A sort-of explanation, at least to connect to some little examples: a subgroup of a reductive $p$-adic group $G$ is "special" or "good" if it fixes a "special/good" vertex in the affine building. A vertex is special/good when it is special/good in every apartment in which it lies, in the building (in the unique maximal apartment system). Each apartment is an affine Coxeter complex attached to Coxeter group $(G,S)$ with generators $S$. A vertex in an affine Coxeter complex is special when its stabilizer surjects to the "linear parts" quotient of the affine Coxeter group by its translations. That is, the fixer of that point surjects to the spherical Weyl group of $G$.
Examples: all maximal compacts in $SL_n(\mathbb Q_p)$ are special/good, because all vertices look the same. E.g., $SL_3(\mathbb Q_p)$'s apartments are simplicial complexes that look like the equilateral-triangle tesselation of $\mathbb R^2$. But for $Sp_4(\mathbb Q_p)$ (four-by-four matrices) there are two different types of vertices: the apartments look like planes tesselated by squares with diagonals added touching vertices $(m,n)$ with $m,n$ of the same parity (is one way to describe it in words...). Thus, half the vertices will have the vertical and horizontal $1$-simplices and four diagonals connected to them, while the other half of the vertices have only the four vertical and horizontal simplices connected to them. The former are special, the latter are not special.