Simplify the following text for a Grade 10 native English-speaking person.
Original: The Second-in-Command ( 2i\/c or 2IC ) is the deputy commander of any British Army or Royal Marines unit , from battalion or regiment downwards .
Simplified: The Second-in-Command ( 2i\/c or 2IC ) is the second commander , the person that is in charge after the commander , of any British Army or British Royal Marines unit\/group .