The technical capacitor model presented in the introduction page on the capacitor models shows that the impedance real part of a technical capacitor is due to the capacitor series resistance rs and to the capacitor parallel resistance rp.
The contribution due to rs is important at high frequency
while the contribution due to rp has a dominating effect essentially at low frequency,
in particular at the industrial frequency.
The losses are given by: PL = rsi2 + rpIR2.
In order to simplify the calculation, an "Equivalent Series Resistance" is used insteed of rs and rp. The theoretical presentation is given in the link below.