dQ is the heat energy supplied to an ideal gas under isochoric conditions. If dU and dW denote the change in internal energy and the work done respectively then
Under isochoric (constant volume) conditions, the heat energy (dQ) supplied to an ideal gas contributes solely to the change in its internal energy (dU). Mathematically, this can be expressed as dQ=dU+dW ⇒dQ=dU‌‌(∵dW=0)