I am no expert on this, but this is my understanding of it:
A Red Sector on a navigation
chart marks a dangerous area or location (or a dangerous approach to a channel or pass, etc. ). In short, most vessels should not go through that area. Possible reasons: rocks, shoals, reefs
, wrecks, etc.
It is always best to understand these things using a good official reference, such as:
A US Chart #1 will have more info on it. You can download a copy here for free:
U.S. Chart No. 1
Hope that helps!
P.S. Just saw that others have answered while I was helping another friend, before I could post, and those others have many miles of great experience.