When one teen is snubbing another Sim, they are completely ignoring them: they won't go to parties they are invited to, they'll push the other Sim away in interactions and they will ignore them until their relationship goes below zero. The Sim that is being snubbed will get the 'Being Ignored' negative moodlet when they are near the teen who snubbed them.
'Start snubbing (name)' is a common wish when a teen is having a mood swing. If snubbing is a promised wish, it will not go away until the teen's mood swing is over. Overall snubbing can be fun to do to a parent when they are a pain towards your Sims.