“After you.”

You're walking into a building, and there's someone walking behind you. You open the door, and you hold it so that the other person can go through first. You want to tell that person to go through before you, so you say:

After you.

After you.

This is a polite phrase that you say to tell someone that they can go before you. Use it when you're holding a door open for someone, or when you're in a line for something and want to let someone go before you.