Like make them rounded.
ETC: You have a cube. The edges/vertecies(The lengths of the edges) become rounded, like as of most things, like boxes, etc.
Like make them “rounded”
search for “Smooth vertex” in the toolbar, edit mode.
W > Smooth or the Smooth Vertex button on the Toolshelf on the left (T to toggle the toolshelf on/off).
You might also be asking how to make them /look/ smooth. In that case select your vertices and w->shade smooth.
I am not sure whether I understand the original question, but maybe you can use Bevel modifier for it.
You could always use edgeloops or extrusions and then use the “Smooth Vertex” tool.
Maybe you are referring to subsurf modifier?
Select cube Ctrl+2 (if keys are default)
Or go to modifier panel and choose add modifier > subsurf