It all depends on your perspective!
If you are talking in general about your AS, then the answers that Jitendra and ErickB gave are spot on.
If you're talking about router-to-router within your own AS, then things are a little different.
Remember that routing takes place based on local decisions per router. So if I start changing the local pref, and you get that, I can influence the path selection that you'll take towards me (incoming) as well as my own decision for going out (outgoing) can be made.
Local Preference is a non-transitive value though, meaning that whatever you set will only be passed to other speakers with your autonomous system.