The Electric Meter is a tool from Railcraft. It is used to measure the Railcraft Power in the various machines that use/transfer it, including the Electric Locomotive, Electric Feeder Unit, various Electric Tracks, and the Electric Shunting Wire. It will not measure non-Railcraft Power machines, such as IndustrialCraft 2's machines.
Right clicking on a block that uses the Railcraft power will output various information to the in-game chat. This information includes the loss-rate and the charge. The first value is the charge that the block has. The second value is the maximum charge that it can have. The final value is how much loss the block has when transferring energy.