Venues in which reporters are permanently assigned are referred to as Beats.
In simpler terms, it is an area of specialization where a journalist has developed reliable sources and has proven his expertise well by publishing stories.
Basically it is like defining a particular category for a journalist. for example- a person who is consistently covering news related to sports then such journalist shall be said to have the Sports beat.
There are various beats in journalism, some of them are Political journalism- which deals with stories related to politics, Civic journalism - which covers stories related to citizen's wellness, Sports journalism- deals with stories related to sports, Food journalism- deals with what food is trending nowadays.
It must be noted that beats are different from types of journalism.