The equation you're interested in describes the energy (in the form of heat) needed to change the state of a substance from solid to liquid.
#q = m * DeltaH#, where
#m# - the mass of the substance;
#DeltaH# - the enthalpy of fusion, which describes the heat required to change the state of 1 gram of a substance from solid to liquid.
Notice that the equation lacks a term that describes a change in temperature (#DeltaT#); that happens because the enthalpy of fusion is a latent heat, i.e. the heat absorbed by a substance during a constant temperature process.
In your case, the enthalpy of fusion applies to substances at their melting points.
Here's how you'd use this in a problem. Let's say you want to calculate the energy required to transform 20.0 g of ice at #0^@"C"# to liquid water at #0^@"C"# #-># perform a phase change.
The enthalpy of fusion for water is listed as #"334 J/g"#. This means that you need to provide 334 J for every gram of water to get to go from solid to liquid at constant temperature.
#q = m* DeltaH#
#q = 20.0cancel("g") * 334"J"/cancel("g") = color(green)("6680 J")#
That's the energy 20.0 g of water needs in order to go from solid (ice) to liquid.