What different between this color code ( #fff3f3f3 ) ( #ffffff ) the first one is 8 characters and second one is 6? [duplicate]

This question already has an answer here:

• Convert RGBA to Hex or ARGB to Hex 1 answer
• ARGB Hex color not working in css html 2 answers

What the difference between this color code #fff3f3f3 and #ffffff - the first one is 8 characters and second one is 6 ?

And how I can read that kind of colors #fff3f3f3 ?