We use base-10 (decimal) for everyday counting, but computers think in binary (base-2). Programmers also use octal (base-8) and hexadecimal (base-16).
Each base uses a different set of digits. Binary has 0 and 1. Hex has 0-9 and A-F. It's all just different ways of representing numbers!
Base converters let you input in any base and get output in any other. Essential for programmers!