Complex sentence: A bignum system in a computer or program allows internal representation of arbitrarily large integers or arbitrarily precise rational numbers, such as by storing numbers as ratios of digit lists which can grow using dynamically allocated memory.
Split into simpler sentences: A bignum system in a computer or program allows internal representation of arbitrarily large integers or arbitrarily precise rational numbers. Numbers are typically stored as (ratios of) digit lists which can grow using dynamically allocated memory.