Binary numbers are commonly found in math or computer languages. Binary refers to a base-2 number system. Usually, the two numbers that are included in the binary number would be zeroes and ones as in the case of 11011. Computers use binary numbers because older computers operated on an on and off system.
Also, there are ways to convert binary numbers into other base numbers. For instance, an octal has a base 8 and hexadecimal has a base 16. These are shortcuts for the binary numbers.
Both of these types have powers of two. If you want to convert the binary number 11011 to a hexadecimal, you would need to perform a set of steps in order to get 1b as your hexadecimal.