High electric fields are often present in some laptop computers when plugged into their charger/power supply. These power supplies are often "double insulated", which means they do not have a safety earth connection. Unfortunately this means that the low-voltage supply to the laptop then "floats" to typically half the mains supply voltage (i.e. about 115 volts AC in the UK and Europe). There is not enough current to electrocute you, but it does expose you to high levels of electric fields of several hundreds of volts per metre at your hands and lap. This is high enough that many sensitive people report adverse effects from some laptops.
The earthing lead is designed to make contact between the mains earth and the metal chassis of the laptop, which minimises the electric fields produced by the laptop.
Please note that it does require the use of either one USB port and whilst the earthing lead is in place these ports cannot be used for their normal purpose.
We offer as standard the Laptop Earthing Lead with a 3-pin UK mains plug. We also offer an option without a plug, for countries outside the UK, which you can wire with your own local style of mains plug.