This one is really not very difficult but there is endless confusion regarding its interpretation.
The first viewpoint, and in keeping with the 'negative' education on this planet, is that one functions purely for self, which is recognised to be the perceivable personality, the conscious/subconscious mind and ego (the lower self). There is no real recognition of higher-self, or God, or higher purposes. This is serving the (ego) self. A second viewpoint is the Christian one of serving God ('Thy will and not my will").
Up to a relatively short while ago these were the main two viewpoints. But we find that neither are satisfactory. Serving self is totally materialistic, is Darwinian, of Newtonian physics, a world of out-of-phase energies and randomness, which we find leads to destruction eventually of the whole. We soon sense there is something wrong with the simple viewpoint.
The Christian view of serving God also has its weaknesses. It is, however, the next step above the serving self interpretation. There is a recognition of power but a power which humans won't take responsibility for, and thus assign it to external causes. This attributing of power may apply to anything from a ritual, to psychic healing, to medical science. It is a way of getting the power to operate by not taking responsibility and believing it comes from elsewhere. This enables the power to operate---unconsciously.
We create an objective world by denying responsibility and this, amazingly, enables us to do this without believing in ourselves (our power). Even an extreme objective event such as a combustion engine will only operate through basic original agreements (all is fundamentally subjective) but by denying consciousness has anything to do with it, it becomes extremely objective and separate, and seemingly independent.
What we are stating is that the power in humans operates indirectly (due to lack of responsibility) so that one doesn't know it is coming from oneself. Thus the concept of 'serving God' as an external source is also the result of not taking responsibility. But it can still work this way up to a point.
The best solution is neither emphasis on ego-self or God (externally), and recognising that the energy creating self, which is under continuous creation from the source (God), is the real-self, or God-self, an aspect of the One in each human (and each atom, etc.). The ego-self is a structure which builds up in the 3rd dimension and begins to function its own way according to a very narrow span of data, and goes out of phase with the higher-self, real-self, or true-self, which is an aspect of the God energy.
Thus one is only serving self (in the true sense) even when it is God. Some people write this 'Self' with a capital 'S'. It is not logical or intelligent to serve someone else. One acts as the greater self because it preserves and expands the whole and automatically operates for the greatest good. The ego-self destroys the whole ultimately.