Simplify the following text for a Grade 7 native English-speaking person.
Original: In physics , Planck units are physical units of measurement defined exclusively in terms of five universal physical constants listed below , in such a manner that these five physical constants take on the numerical value of 1 when expressed in terms of these units . Planck units elegantly simplify particular algebraic expressions appearing in physical law .
Simplified: The Planck units are physical units of measurement .