The most important thing is a predictable structure. Let's say you've never heard Haydn's Surprise Symphony. When you hear that chord, it will distract you and break your concentration for sure! Other than that, if you have heard other Haydn symphonies you know what to expect, and your subconscious mind doesn't have to keep bringing things to your attention.
When composing music, you need to know what kind of music your listener is familiar with. An old person who has never listened to punk rock will find punk rock distracting. A young person who is not used to rubato will find Chopin distracting.
The second consideration is that the music should mask out other interruptions. Suppose a dog barks outside. That grabs your attention. However, if you are listening to music, your subconscious mind hears the bark as less worthy of attention than the music, which is already not worthy of attention.
Movie music that is intended to be a background to a conversation limits the amount of sound energy in the normal speech register (say the two or three octaves starting at C below middle C) so that the speech sounds are clearly heard. Background music that is intended to cover up speech (and let you work better when many people are talking) should concentrate more sound energy in this range. The clarinet could be a better instrument than the violin for this, because the sound energy of a clarinet note is more focused in a narrow range in the normal speech register.
Music with regular, predictable rhythms may make unpredictable background sounds stand out more. If you are used to listening to the last section of Stravinsky's Le Sacre du Printemps with its constantly-changing irregular rhythms, a piece written in a similar style would cover up those background sounds.
