I've been looking for an easy to understand explanation of the folate trap and came across a couple of great articles. There's still quite a bit that is beyond me, but the first line says it all (and explains why you should take B12 and folate together):
The second article is a great summary of folate deficiency itself:
Suspected hypothyroid now suspected b12 and folate deficiency
Folate
B12 and folate deficiency 
Ferritin folate vitamin D and vitamin B12
