The ratio, $r$, is the fraction of the particles of the material 1 in the mixture, e.g., 0.1 means that each 10 particles in the mixture contain 1 particle made of material 1 and 9 particles made of material 2; 0.9 means that among 10 particles 9 are made of material 1 and 1 is made of material 2.
The step in ratio denotes the values for $r$ which will be considered, e.g., a step in ratio of 0.1 consideres $r \in \{0.0, 0.1,0.2,0.3, \cdots 1.0 \}$