Hexadecimal uses sixteen distinct symbols, in the case of css color the symbols 0–9
to represent values zero to nine (obviously), and A, B, C, D, E, F
to represent values ten to fifteen. So, using one Hexadecimal character you can represent 16 values. With two Hexadecimal you can represent 256 (16*16) values.
In RGB
you have colours represented by Red Green Blue (R=0-255, G=0-255, B=0-255), so we use 3 pairs of Hexadecimal symbols! So when you see an RGB color, you can make the calculation below.
Example:
Hex:
#4C8ED5
is RGB:76, 142, 213
.
Because 4C = 76
(Red), 8E = 142
(Green), D5 = 213
(Blue)!
Hope it helps your understanding!
More to read: Hexadecimal on Wikipedia and a nice RGB to Hexidecimal Converter