A mesh is vertices and triangles: an explicit surface with connectivity, what renderers, physics engines and grasp planners take.
It has no free space and is awkward to update, so maps are fused in a grid and meshed afterwards.
Marching cubes: the signs at a cube's 8 corners give 256 cases, 15 up to symmetry, each a few triangles.

Image: Trevorgoodchild, public domain, via Wikimedia Commons

Where the surface cuts an edge with values $\phi_a$ and $\phi_b$:

\[t = \frac{\phi_a}{\phi_a - \phi_b} = \frac{0.6}{0.6 - (-0.3)} = 0.67\]