Not sure whether I'm smarter than you but I have studied EU Law. There are provisions in the EU Treaties that give citizens of member states an "EU citizenship" (see Art. 20 TFEU). Those EU citizenships give EU citizens – among other rights – the right to live and work in any other EU country and vote in local elections [1] Before Brexit British citizens had this EU citizenship as well.

The question that arose after Brexit was, whether British citizens would possibly retain this citizenship, which could have meant that they would retain the rights that come with it as well. There are different arguments why British citizens might have kept this EU citizenship, for example it might be argued that the withdrawal of a _state_ from the Union can not mean that _individuals_ loose their rights or it could be argued that such a loss of rights would be disproportionate.

The CJEU has now interpreted the treaties in such a way that the withdrawal does indeed mean that individuals loose their EU citizenship, arguing that "the authors of the Treaties thus established an inseparable and exclusive link between possession of the nationality of a Member State and not only the acquisition, but also the retention, of the status of citizen of the Union".

So this ruling might have had a big effect if the CJEU decided in a different way. The way they decided now, everything stays the same. If you are in possession of another EU member states citizenship (Slovakia) this will not change anything for you, you will remain an EU citizen.

[1] Technically many of those rights were already in the treaties before the introduction of the EU citizenship, so it is a bit "declaratory" in nature.