This is SCT 013 000 100A Non-Invasive AC Current Sensor Split Core Type Clamp Meter Sensor. SCT-013-000 is a Non-Invasive AC current sensor i.e. it is a current transformer which can be used to measure AC current up to 100 amperes.
Current transformers (CTs) are sensors are for measuring alternating current. They are particularly useful for measuring whole building electricity consumption (or generation for that matter). The split core type such as the SCT-013-000 CT in the picture is particularly suitable for DIY use. Clip this SCT 013 000 straight on to either the live or neutral wire coming into the building without having to do any high voltage electrical work.
Like any other transformer, a current transformer has a primary winding, a magnetic core, and a secondary winding. In the case of whole building monitoring the primary is the life or the neutral wire (not both) coming into the building itself and goes through the hole in the CT. The secondary winding comprises many turns of fine wire housed within the casing of the transformer.