If you live in a Conservation Area, the conditions to meet to obtain Planning permission are stricter, as the area you live in has special historical or architectural characteristics that are viewed as worth preserving.
Although conservation areas have some extra Planning controls, these exist to protect what makes the place special. They are most likely to affect owners who want to work on the outside of their building or any trees on their property.
This doesn’t mean that you can’t build, but rather that you have to take into account these stricter controls when you do.
Historic England provides useful information on Conservation Areas in the UK, here.