When a electrical current of I amphere passes through a resiator R, electrical power equal to I2R is dissipated in the resistor.
This dissipated electrical power appears in the form of heat and resistor is heated up.
Physicaly, resistance resist the current flow. Actually flow of current which is flow of electrons encounter collision with
electrons and atoms of resistive materials and energy is transferred. Hence electrons and atoms in the resistive material gains energy.
Electrons speed in resistive materials increases and atoms vibrate with more amplitude.
These effect results to increase the temperature and we observe that due to passing of current, resisting material is hetaed up