People who are important but only within their limited circle of influence.
The converse phrase is of course 'small fish in a big pond'. The phrases are often used to convey the degree of ambition a person holds. There are fewer 'big fish' and they have kudos and power locally. Those with wider ambitions swim amongst the more numerous but relatively less influential 'small fish'. The implication behind labelling someone 'a big fish in a small pond' is that he/she is content to stay in that position whereas the 'small fish' have the chance to become 'big fish in a big pond'.
The phrase is American and the earliest reference I can find to it is in The Galveston Daily News, June 1881:
"They [local vested interests in Galveston] are big fish in a small pond."