Don't know what is it used for,and how it operates?I don't have any instructions for it.

You posted under T101 timer.
Most box timers have model number and instructions pasted on inside of door.
Instructions can seems a foreign language sometimes.

If your timer is different than T101, then add a comment with timer number.
I have extensive files on timers.

Box timers are generally used for heavy amperage loads like outdoor lights, swimming pool pumps, water heaters, parking lot lights and lighted outdoor signs.
They are surface mounted near breaker box, or in attic or basement, or garage.
Box timers are used in schools to ring bells, and inside factories to run processes and fans.
Some are complicated electronic devices used to control machines with one second precision.
Mechanical box timers are noisy when turning on-off, so are generally not installed in living area of house.

T101 timer is least complicated of any timer.
It is used for ordinary household 120V circuit.
Timer has 120Volt clock motor, and controls 120Volt circuit.
For example 120Volt water heater, or 120V grow lights, or aquarium, or any application up to 2 horsepower, for example timer can be located inside garage to control outdoor lights.
T101R is raintight outdoor box, with rubber gasket on door that seals tight when closed.
Raintight boxes are installed outdoors to operate 120Volt spas and pool pumps.
When box timer is rated for indoor, there is no rubber gasket on door, and that box is called NEMA 1 enclosure.
My local Lowes sells T101 T103 T104 > and each is for slightly different purpose and each wires differently.
Link below shows wiring and manuals for T100 series timers.
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.html#T101

Here's how T100 series timer works:
Lift and rotate yellow dial until current time lines up with silver pointer.
Clock motor rotates dial as time passes.
Clock motors are accurate for years, but current time has to be reset after power outage.
ON-OFF trippers are placed on outer edge of dial.
Trippers push a lever, and lever turns circuit on-and-off.
ON tripper pushes lever one direction.
OFF tripper pushes lever opposite direction.
Each time lever is pushed, manual-override lever clicks back and forth with audible sound.