It's 90% posturing and 10% substance. There was a nationalist frenzy in the 1990s (government induced but also some sort post-cold war national panic) regarding the fact that the Former Yugoslav Republic of Macedonia wanted to be known as Republic of Macedonia - this led to a very widespread (and toxic) nationalist discourse that has made it very costly politically for any Greek government to accept even a compound name for RoM (i.e. Northern Macedonia, Upper Macedonia, New Macedonia).

However, there are some marginal real issues here, minor ones that could be settled with some goodwill, such as irredentism in the RoM (a political minority but firmly entrenched in school curricula and public discourse there) and product names (all things exported as "Macedonian" from Greece, or public institutions would be forced to change their name if there is no other arrangement).

Both of these issues could be settled by no more that a month's sincere discussions, and some sort of arbitration from the EU - but positions are entrenched now and both countries will have a hard time bargaining... So the whole thing degenerates into discussing the "nationality" of Alexander the Great instead...

The road of excess leads to the palace of wisdom - William Blake