What does being 2 faced mean?

Like you tell your friend she is pretty and she is your best friend and you lead her to believe you can be trusted and then you go telling others the opposite.

It is a pretty literally translated saying. It just means to show two faces. Like if you have one friend who the rest of your friends don't like and you are nice to that fried, unless your other friends are around. THe same thing as "backstabbing".

Here are a couple of other examples:

Telling a friend that you really like him or her and then telling others that you don't like the friend that you have told that you like.

People going to church and pretending to others that they are living lives according to what they are being taught in their church and then living their lives outside of their church the opposite of what they have been taught and agreed to do outside of attending their church.