In more detail
Each column is worth 16 times the one to its right (1, 16, 256, 4096...), and the letters A to F stand for 10 to 15. One hex digit is exactly four bits, so a Byte is always two hex digits (00 to FF) and a 32-bit value is eight. Hex appears in colour codes (#FF8800), MAC addresses, IPv6 addresses, hash values, memory dumps and file signatures. In code it is often prefixed 0x; in CSS and HTML colours it follows #.
Why it matters
The same digits mean different numbers in different bases: 0x10 is 16, not 10. Hex values are case-insensitive (ff = FF), but a missing leading zero shifts every following byte, which is a common mistake when typing colours or MAC addresses by hand. Reading raw bytes in hex is the usual way to identify a file type or spot hidden characters.
Example
CA in hex is 12 × 16 + 10 = 202 in decimal and 11001010 in binary. The colour #FF8800 splits into the bytes FF, 88 and 00, which are red 255, green 136 and blue 0. The text Hi is stored as the bytes 48 69.