Complex sentence: In the standard model of AGN, cold material close to a black haccretion disc transport matter inwamomentum outwards, while causing the accretion disc to heat up.
Split into simpler sentences: In the standard model of AGN, cold material close to a black hole forms an accretion disc. Dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up.