A variant spellcasting class for magic-users with more informal training - they get less potent spellcasting, but more baseline magical abilities, plus some fighter-inspired elements (like wizard school-based fighting styles). The mage-errant also has three subclasses: the Knight, more heavily a fighter-mage (like the 5E official eldritch knight or bladesinger); the Infuser, basically a take on the 5E artificer; and the Glyph, which focuses on rune magic. The product concludes with a handful of new spells (Lightning Recoil probably being my pick of the lot).

Overall, the class and its subclasses have some interesting abilities, though a number seem a bit more complex than they had to be. The core class could probably fit into most D&D 5E settings, though it honestly doesn't feel as archetypal as any of the official spellcasting classes, which suggests the main appeal would be for the mechanics rather than the flavor. The Knight and Infuser also don't offer much conceptually that you couldn't get from official counterparts, though I could see stealing some of the Infuser's Wonderous Creations for the official artificer. The Glyph, on the other hand, does represent an archetype not adequately represented thus far in official 5E, and is the highlight of the three from a creative standpoint.