Sentence:  <WSD>Forests</WSD> are considered the security boundary in Active Directory; by this we mean  that if you need to definitively restrict access to a resource within a particular domain so that administrators from other domains do not have any access to it  whatsoever, you need to implement a separate <WSD>forest</WSD> instead of using an additional domain within the current <WSD>forest.</WSD> 
Meaning of the word marked with <WSD> tags:  A group of domains that are managed as a unit.