Complex sentence: Electric fields are caused by electric charges (described by Gauss's law) or varying magnetic fields (described by Faraday's law of induction), which together are enough to define the behavior of the electric field as a function of charge repartition and magnetic field.
Split into simpler sentences: Electric fields are caused by electric charges, described by Gauss's law, or varying magnetic fields, described by Faraday's law of induction. Together, these laws are enough to define the behavior of the electric field as a function of charge repartition and magnetic field.