I am trying to understand the marching cubes algorithm. I would like very much an easier definition of an isosurface than what is available online. Could anyone please explain it?
Isosurface is another way to call a surface defined by the implicit equation
$$F(x,y,z)=f$$ where $F$ is a function of space and $f$ a constant, often $0$. The prefix iso- indicates that the function $F$ takes the same value ($f$) all over the surface.
The marching cube algorithm is able to construct the iso-surfaces (decomposed in triangles) for a given $F$, by sampling over a regular grid.
It's a 2d/3d function where a ouside surface is defined as everywhere in the graph where the value crosses a certain point for example from 0.999 to 1.0001, 1 is the isometric value... <1 is inside the surface and >1 is outside of it. if the surface is the formula surface = y+1; then it is a flat plane parralel to the y axis. Search for isosurface tutorial/guide on google to have illustrated learning pages.