The idea behind the term 'lumped' is that we take a single value to represent some property or parameter that is actually distributed or variable in space.
This idea is used in several branches of engineering to solve the controlling (differential) equations.
In heat ransfer the assumption is that temperature within a control volume is even, at any instant. Even does not mean constant, just that there is no temperature variation within the control volume.
This assumption allows us to use the equation
Input minus Output = Accumulation minus Internal Generation
in differential form with respect to time.
So if we plunge a hot workpiece into a quenching vat and assume the temperature of the whole workpiece to be one temperature, T then the rate of cooling is found by solving the differential equation that arises from equating the heat transfer across the boundary to the rate of temperature change of the workpiece
[tex]- hSurfaceAre{a_{workpiece}}(T - {T_{vat}})\quad = \quad Mas{s_{workpiece}}{c_{workpiece}}\frac{{dT}}{{dt}}[/tex]
There are quite a few web presentations about this
look up the lumped capacitanc method
you should also look up the Biot Number.