Turbulent dissipation is the viscous conversion of mechanical energy to heat.
Imagine putting energy into a fluid system by dragging your hand through it. Behind your hand, you see circular vortices forming. These vortices spin off smaller vortices which in turn spin off even smaller vortices. In this cascade, kinetic energy is carried down to smaller and smaller scales. At a particular scale, viscosity smooths out the flow, stopping the cascade and converting the mechanical energy to heat.