You read that right, this is a section under Allegory that is also about allegory. That's because Allegory is itself such a crucial and defining tool for Spenser and his poem. Allegory, similar to personification, is the practice of imagining characters and places as direct embodiments of a virtue, value, idea, concept, etc. For this reason, a lot of the important Images, Allegories and Symbols in The Faerie Queene you'll find in the section on Characters, because so many of the symbolic qualities of the poem are articulated through its super-allegorical characters. So hint: check out the section on Characters for a wealth of info.
So, while all literature involves endowing characters with larger thematic oomph, or crafting spaces that suggest more than just the space itself, allegory takes this to a whole new level, and is often, though not always, really obvious about it. So while Hamlet, for example, might be said to embody inaction or self-doubt, those are qualities we see him manifest as part of a larger collection of character traits as opposed to totally and completely defining his character.
In Spenser, by contrast, the majority of characters are what they embody and, often what they are called. So Error, is, pretty much, just the concept of Error—this monster doesn't develop, take on new traits, or become particularly nuanced at any point.
However, just to make things a little more interesting, Spenser doesn't always make what his characters embody as clear as "Error." His major characters in particular tend to be more complex and multifaceted than the minor ones, so that while we know Britomart is a figure deeply associated with both chastity and Britain, she does more in the poem than just be chaste—she figures out what chastity means, makes mistakes, and sometimes has adventures that don't clearly relate to her status as chastity at all.
So while The Faerie Queene is absolutely an allegory, it's a complicated allegory. Indeed, some people have read the poem as—get ready for it—an allegory of allegory itself. That "ka-pow" sound you just heard was your mind being blown, Shmoopers.
The Faerie Queene is a poem that is thinking through the very nature of allegorical meaning, literary meaning, and the power of representation. Chew on that.