Does the term "series opposing" mean that sources are in series with opposite polarities?
Yes. You are correct sources connected with opposite polarities.
When two or more than two emf sources in a circuit make the current flow in the same direction, these are called to be connected as Series Aiding. The voltages sources are connected end-to-end, positive-to-negative, and voltages of all sources are added.
When sources of emf that force the flow of current in opposite directions are placed in a circuit, these are said to be Series Opposing, and the effective voltage is the difference between the opposing voltages. It has no practical use.
In this case, when two opposing sources are inserted in a circuit the direction of the current flow would be determined by the voltage of the larger source.