f00

In programming, F00 is what you call a variable with a vague name in which you store a color.
#F00 is the color code for bright red in simplified rgb hex. Foo is a variable name chosen by lazy programmers who can't come up with better variable names.
Plural: f00s
Verb: f00ing

Person 1: I just made this javascript code:

var meh = "#FFF";
var iCantThinkOfWhatToNameThis = "#BADA55";
var F00 = "#F00";
var bar = "#123456";
Person 2: Stop f00ing your code up with so many f00s.