So as it states above, EMF stands for 'Electro Magnetic Field' or 'Electro Magnetic Frequency'. Electro magnetism occurs when electric currents move or change. The stronger the field or frequency the higher the reading... and whilst you can't see these fields they are prevalent in the atmosphere from natural, electrical and magnetic sources.
Common sources of EMF are from all electrically powered equipment like audio, video, cell phones, power lines and transmitters. Natural sources are from the atmosphere charging as in a thunderstorm when lightning strikes.
In Paranormal Investigation
Many Paranormal Investigators choose to use EMF meters as part of their equipment and tools for the detection of EMF fields that might be flooding locations. Many modern buildings all have electric wiring and distribution boards in key locations throughout a building, so it is a must for an investigator to establish these readings to mark out where they are during an investigation.
The main reason for these pre-readings (often called 'base-line') are for detecting any fluctuations and changes in the environment that [might] identifies [possible] paranormal activity. But it is with this information that some Paranormal Investigators can misuse the EMF meter. Base-line readings must be taken with and without lights on... and must always ensure radios and other equipment is checked against the meter to ensure no anomalous readings will be detected and identify false-positives.
Paranormal Investigators use the EMF meter to identify possible large fields or frequencies that might be the reason for alleged paranormal activity and phenomena. In fact evidence suggests that large EMF readings can cause a surge in melatonin (the chemical that regulates sleep in the human body) and so can cause sleeplessness - which can make some people believe they are experiencing paranormal activity.
Other people can be sensitive to EMF and therefore can experience headaches, nausea, hallucinations and feelings of dread and fear.
Ghosts and EMF
Due to popular TV programmes that depict paranormal activity (like Most Haunted, Ghost Adventures, etc.) there is a huge mis-conception that EMF meters 'detect' ghosts. This is not the case at all. In fact there is no scientific evidence to suggest that this is correct.