The air is almost thick when they are in a room together, it's almost electric or intoxicating. It's when there is a strong sexual chemistry between two people that cannot be satisfied. It may be so strong that they are almost embarrassed or nervous to be around each other and avoid one another because they are afraid their lust might come across in conversation or interaction. It may be very awkward between them because most of the time the tension arises from one or both parties being unable to persue a sexual relationship with the other because they either work together, are already in relationships with other people, or there is something else preventing them from uniting sexually so they need to play it cool. Coversation is often stilted and uncomfortable. So there is a strong desire to be around the person and see them, but they also want to avoid each other thus avoiding any awkward interaction or frustration. Most of the time both parties already know the other wants to hook up but it can't happen or shouldn't happen...
There can also be sexual tension among friends but the awkwardness isn't so much there because they already know each other.