This shows the skeleta of the simplex $\Delta_3$. The full simplex
is the set of points $(x_0,x_1,x_2,x_3)\in\mathbb{R}^4$ where
$x_0,x_1,x_2,x_3\geq 0$ and $x_0+x_1+x_2+x_3=1$. The $k$-skeleton
is the subspace where at least $3-k$ of the coordinates $x_i$ are
zero.
The $0$-skeleton just consists
of the four vertices of the tetrahedron.
The $1$-skeleton consists of
the four vertices together with the six edges joinng them.
The $2$-skeleton consists of
the four triangular faces of the tetrahedron together with their
edges and vertices. However, the solid volume in the middle is
not part of the $2$-skeleton.
The $3$-skeleton consists of the full solid tetrahedron.