It's complicated - but a high-level and incomplete version:

Sweden granted asylum to some Kurdish asylum seekers, and banned sales of weapons to Turkey over the use of force against the Kurds in Syria.

Turkey believes that the Kurds that were given asylum are terrorists and said they won't agree to Sweden's NATO bit until the Kurds are turned over to Turkey.

The Swedish government actually agreed to hand them over at one point, but the Swedish courts have ruled that handing the Kurds over would be illegal under asylum laws in Sweden.

Since Sweden can't turn them over Turkey has started a massive campaign to try and frame Sweden as racist against Turks (for domestic political reasons) and as supporting terrorists.

The down side for Turkey is that in the long run they are going to a price for keeping Sweden out of NATO as it is creating a massive backlash against Turkey which will negatively impact them both economically, diplomatically and militarily going forward as they are increasingly seen as an unreliable ally.