What is exciting current in electrical machines?

In alternators you have excitation coil (windings). Now a days this we see synchronous brush-less alternators. Hence rotor of these machines is used to excite the stator windings from where the output is obtained.

AVR is used to input the excitation voltage (D.C) to the excitation windings. Change in this voltage changes the output voltage drastically. Hence, the current in excitation windings called the excitation current.

The DC required to magnetize a piece of metal is defined as excitation current!!! how it effects even i dont know!!!

To generate electricity from generators, we have to use mechanical energy for rotating the rotor. Nowadays, we use alternators to generate electricity.
Now, the basic principle we use to generate electricity is the Faraday's laws of electromagnetic induction. We basically move a conductor through a magnetic field to produce an emf in that conductor or we move a magnet relative to the conductor(keeping the conductor stationary or otherwise) to produce emf. We use this emf to produce a current and we use it. Now, in alternators producing power in the value from a few kilowatts to a few megawatts, we need a large magnet . As using a large magnet is totally uneconomical, we use an electromagnet in its place. The alternators have rotors which have exciters which are basically electromagnets. The current we give to the exciter is the exciter current. This current is responsible for the value of voltage and current, that is, the power output of the generator.
Hope this answer has helped you. Thanks for reading it.

=ashuashi;

AVR is used to input the excitation voltage (D.C) to the excitation windings.

With due respect, kindly let me point out something here. What I believe is that an AVR(automatic voltage regulator) is rather a controller of excitation than a producer of excitation. You definitely haven't meant to say so, but the placing of words is such. I personally felt for a moment that you meant that way. For anyone new to electrical basics, this can be really confusing. Thank you

For others' information-
There is either a DC generator or an AC generator and rectifier arrangement coupled to the shaft of the main generator which produces the excitation. The AVR control its output by controlling the DC supply to its exciter, which can be given from other sources.