Decimal (base-10) uses ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. This is what you learned as a child and use every day.
The number 3,742 means , which equals . Each position is a power of 10, starting from on the right.
Why base-10? Humans have ten fingers. Other bases follow the same logic but use different numbers as the foundation. Now let me show you base-2, where everything changes.