Sections help in managing your Content tree and permissions.
When managing your Content tree, you can designate any number of Content items as a separate Section. You can then use Sections to determine which User or User group has access to which part of your Content.
A User (or User group) can be assigned Policies which allow them to use any set of Sections. You can also grant Policies only for particular operations on Content: for example you can give a user the permissions to edit one Section, but only to view another.