When things feel so tight they might snap, that's tension. If you buy your girlfriend a vacuum cleaner when she wanted diamonds, you will experience tension. Just before she storms out of the room.
The noun tension has its Latin roots in tendere, which means to stretch, and tension occurs when something is stretched either physically or emotionally. Strained relations between countries can cause political tensions to rise. You can add tension to a rubber band by stretching it tight. You can release nervous tension by releasing that tension in the rubber band, when you shoot it at your brother.