This is a commonly used acronym to refer to "electromagnetic frequencies". It's often used in the context of talking about wireless radiation.
To confuse matters, some people refer to wireless radiation somewhat more accurately as "EMR" standing for "electromagnetic radiation". But since I've heard "EMF" more often, it might be more widely recognized, so that's what I used for the hashtag. Hopefully we can stick with using #EMF, so all the related posts don't get divided into two identical categories of different names...