Symptoms include: slackjaw, wide eyes, a look of awe, a feeling of bewilderment and an overall sense that you may have just been in the presence of a God. In extreme circumstances, the act of fixing ones face has resulted in minor headaches.
It can also be used as a threat.
You see the hottest floosie in the bar and right as you walk up to her she faceplants. You pause then simply say, fix ya face.
If it is not used in a phrase then it is simply a state of being one is in when they see something amazing.