ER is for "extended range". From the UA website:

The first 767-200, designed to be the first wide-body aircraft to fly short and medium routes, was delivered to United in mid-1982. The larger 767-300 series was launched in 1983 and the first airplane was delivered to Japan Airlines in September of 1986. Following the release of both the 200 and 300 series, an extended range version of each was developed to allow the plane to service longer routes.