Take for example your friend says "It's going to rain today" and you tell him it is impossible because the weather is so sunny. If rain happens to fall on that day, your friend will say " In your face". Basically, it is used when two people have different opinions on something and when one of them turns out to be right, he can say "in your face" because he was right all along and the other person turned out to be wrong.

It can be used when you're proven right about something that you've been arguing or discussing with someone. It can also be used when you've beaten someone. For example in a video game, competition or sports. You'd say something like "ha ha ha, in your face!" Usually it's only said in a joking and friendly manner, not with the intention of being mean or nasty.

Moses gives a good example. However, it can be used in any case where the speaker is defying those to whom they are using the expression. The insult is specifically for when the speaker has done something which goes against the intentions or statements of another party. You can even say "in your face, [specific person or other target of insult]" to refer to someone or something not available to hear the insult. For example, if your grade-school teachers said that you were never going to amount to much, when you land your dream job, you might exclaim "in your face, grade-school teachers!"

А какой же все-таки русский эквивалент ?