Timbre is a fancy name for the type of sounds something makes. If you played the same note on say a violin, bassoon and a trumpet they would still be the same note but how the instrument created the sound would be different so the sound of the note would be different. This may sound obvious but when you are creating a particular mood certain timbres compliment that mood. For example a long note played on a violin immediately gives a soothing sound that could be used for romantic or calming moods whereas the same note on the bassoon is more rasping and throaty so it is good for creating tension and fear. The same note on the trumpet is very strong and bell like so it can be used for creating triumph and power.
The next thing is that when you look at the range of an instrument the moods that it creates through its range can change. The violin can be romantic and calming through most of its range but when you get to the highest notes they can become shrill. These notes have been used to create tension as they are hard to listen to and therefore leave us feeling uncomfortable.
So from a composing point of view, especially in film music, the right choice of timbre can mean a lot less work to create the mood that you want.
I have already started a second Music Dictionary dedicated to timbre, production effects, instrument effects and any other things that affect how we hear sound and the moods and pictures that they create in our minds. I will share more of my discoveries as I find them. Enjoy.