A computer is a series of transistor switches that are either on or off. By combining these switches together we can represent numbers, words and operations e.g. on off off on on off on on off off off on on etc etc
In the counting system called binary there are only 2 states i.e. “one” and “zero”. This easily translates to “on” and “off”. This is why computers use binary to represent numbers and words.
Hexadecimal is counting in 16s. Why would we do this?. Hex is a super-useful, short-hand way to represent binary numbers. Instead of writing 10110011 0010111 we can write A32F in hexadecimal – easier to write and less prone to errors.
Here is a useful YouTube video that covers the topic.
Here is a worksheet on the topic.