Prelest is spiritual delusion, deception, illusion, etc. That is what it means.
Here is a pretty good quote from that article...
St. Gregory of Sinai also writes that "around beginners and those who rely on their own counsel the demons spread the nets of destructive thoughts and images, and open pits into which such people fall; for their city is still in the hands of the workers of iniquity, and in their impetuosity they are easily slain by them. It is not surprising that they are deceived, or lose their wits, or have been and still are deluded, or heed what is contrary to truth, or from inexperience and ignorance say things that should not be said. Often some witless person will speak about truth and will hold forth at length without being aware of what he is saying or in a position to give a correct account of things. In this way he troubles many who hear him and by his inept behavior he brings abuse and ridicule on the heads of hesychasts. It is not in the least strange that beginners should be deceived even after making great efforts, for this has happened to many who have sought God, both now and in the past".
I think the important thing to note here is that it is really easy to fall into prelest when you rely on yourself and do not have guidance.
True view of oneself (of own spiritual condition, position relative to God, sinfulness etc.) is tightly connected with the passions of pride and vainglory and is distorted by these passions. The degree of prelest is the degree of such distortion, i.e. the amount of falsehood in the view of oneself and the degree of difficulty of change from the false view to the true one.
Pride and vainglory lead into all manner of prelest, and it is very easy to fall into.
Really, in the broadest sense, we all suffer from some sort of prelest. That being the case, to say that you are completely free of prelest is the surest indication that you are in fact in prelest!
