A P.K.E. Meter measures the Psycho-kinetic energy in the current area. This meter is showing the P.K.E.'s next to your computer. In order to correctly assess the reading, you must view this guide.
200-300 = No Threat, small to no paranormal activity
301-450 = Trouble, you have a faint trace of a spook
451-550 = Biblical Proportions, don't let anyone in your house named "Zuul"
Now that you know how to correctly process the information, now you need to know some other tips. If the meter is reading very close in between 300-305, you may be picking up a trail of a ghoul that was already in your area. In other words, you are seeing his residual traces. If you are reading anywhere from a 450-455 when a ghost is present, you are looking at a Class-5 apparition, a ghost not to be taken lightly.
Now that you know the situation, strap on your pack and jump into action, Ghostbuster!