Using the minipage command, one defines the area to be made available for subsequent commands.  For example, one can typeset in this minipage, Tables, formulae, text, figures, etc.

When we put a figure in this space, we have to tell LaTeX, what size of figure has to be put in.  In other words, the minipage length is like defining the page size, and the second length is the size of the figure that we want to put in.