What does substrate mean?
A substrate is an underlying substance or layer, or less strictly, the medium within or upon which an organism grows. For instance, you can have a filamentous fungus grow on a solid medium (e.g. on an agar plate) or in liquid medium (e.g. in a shaken flask containing a solution) and its substrate would be the contents of the agar or solution. It's nothing more fancy than that.

For instance, you could design 4 growth conditions for a strain of fungus.

Case in point, a hypothetical table could look like this:

                        *Substrate*

                Wheat straw     Sugar beet pulp
                             |
          Liquid         1   |   2         
                             |
*Medium*             -----------------
                             |
           Agar          3   |   4          


Simple!