In 3 phase-3 wire/4 wire,what means of 3 element,2 and 1/2 elemtnt and 2 element system?please explain me.
Thanks.

Search for "Blondel's Theorem." A good reference is Electricity Metering Handbook, 10th edition. Their site is EEI.org.

A simple answer. A watt meter measures the voltage and current in a circuit and displays (calulates) the wattage. Refer to the source provided by Bjarney, for the how/why these systems work.

The elements of a meter refer to the set of voltage and current measuring windings. Therefore:

2-elements can measure two complete sets of voltage and current. They are used on 3-phase Delta as well as on 1-phase systems.

The 2-1/2 element is used to measure three currents and two voltages simultaneously. These are often used on 3-phase 4-wire systems with balanced phase voltages. This construction has some minor inaccuracies (caused if phase voltages are not equal) but they are usually more acceptable than the cost of a 3-element device.

3-elements can measure three independent voltages and currents at the same time. These are used on 3 phase Wye circuits where the phase voltages are not balanced.