Physically it means there are no "fictitious forces" in the frame, in the Newtonian sense, i.e., no centrifugal or Coriolis force. In the absence of gravity we would say that meant the worldlines of free-falling objects are straight lines in the frame, but since gravity is present (due to the Earth), we can't say that. So the ECI frame is really only "inertial" in the Newtonian sense (where gravity is not considered a "fictitious force" and it's OK for the worldlines of objects free-falling under gravity to not be straight lines in the frame). It is not "inertial" in the GR sense (and in the GR sense it is impossible to define a single inertial frame that covers the entire Earth and its neighborhood, as the ECI frame does).