You see hex everywhere in programming. Memory addresses: 0x7FFF5A2B. Colors: #FF5733 (red=FF, green=57, blue=33). Byte values: 0xCA for decimal 202.
The 0x prefix tells the compiler "this is hexadecimal." Without it, CA would be an invalid number or a variable name. With it, 0xCA is a valid integer literal equal to 202.
In CSS, #FFFFFF is white (all channels maxed at 255) and #000000 is black (all channels at 0). Each pair of hex digits is one color channel. Reading hex colors becomes second nature with practice.