Yury Revich violin
(Photo credit: Wikipedia)
A violin is usually not played in a rest position, but it is still important to understand how it can be properly done in an ensemble or during a concert. The reason is that the orchestra is not only an auditory sensation for the audience but a visual one as well, so it's very important to coordinate positions, bowings, and fingerings so that the players look and sound as uniform as possible throughout the entire concert. When every violinist is in a rest position, the orchestra looks like a single unit rather than a bunch of characters casually sitting on stage with instruments.
Rest position simply means holding the violin at your side. You do this by bringing the violin under your arm and keeping it there while supporting the fingerboard with your forearm and hand. You want to press the violin securely against you and should be able to let go with your left hand very easily even despite the fact that it should be holding the neck of the instrument. This is simply because the weight of your arm should be strong enough to support the violin without a struggle.
The appearance of rest position should be uniform in an orchestra setting and should happen at specific intervals to maintain a consistent postural alignment between all the players. When the conductor is not on the podium, all players should be in rest position. When the conductor moves to the podium, take your instrument from rest position to attention position, which is where the violin should rest on the knee. Finally, when the conductor raises his hand to dictate the downbeat, bring the violin up to your shoulder in playing position.
While some aspects of playing the violin are elementary and easy to pick up on such as this one, there are a number of techniques that are extremely challenging and take years and years of work in order to master. For that reason, I always advise every single person interested in playing the violin to get a good teacher and start learning as much from someone who is more knowledgeable than they are on the violin. You will only learn so much from yourself if you try to teach yourself how to play, whereas with a teacher your possibilities on the instrument will be unlocked by learning all the things you don't know that you don't know about the violin.