The unit of the Hubble constant is 1/sec. and it is determining the percentage by which space is expanding each second. The Hubble constant is not constant in time, for this reason it is called also Hubble parameter.
For getting an idea of space expansion you have to imagine a cubic grid throughout the whole space where the grid/ the number of cubes does not change, but each cube is increasing in size. It is similar to a sort of "stretching" of the size of the cubes.
Space includes dark energy whose energy density is constant and does not dilute. This is why in an advanced stage the universe is/ will be dominated by dark energy, which is prevailing/ will prevail over matter and radiation.
Local assemblies of objects in space such as galaxies are practically not concerned by space expansion because their gravitational forces are holding them together even if space is expanding
Space expansion has been observed in the form of redshifting of radiation from very far objects, but at my knowledge there is no current theory explaining how space is created.