Complex sentence: General relativity predicts that gravitational radiation should exist and propagate as a wave at lightspeed: a slowly evolving and weak gravitational field will produce, according to general relativity, effects like those of Newtonian gravitation.
Split into simpler sentences: General relativity predicts that gravitational radiation should exist and propagate as a wave at the speed of light. A slowly evolving and weak gravitational field will produce, according to general relativity, similar effects to those we might expect from Newtonian gravitation.