Usually this refers to the how the gas burner operates. Some boilers are fully modulating which means the gas pressure is controlled electronically keeping the output constant, Other boilers are semi modulating which usually means the boiler has a full rate and a low rate and alternates between the two, A semi modulating combination boiler would result in a non thermostatic shower varying in temperature, also this could be caused if you have poor water pressure or someone else drawing water or the flushing of a toilet.