a box which has resistance between 1 ohm to 50000 ohm

The use of a resistance box or a "Decade Box" is to help calibrate instruments used to measure electrical values such as Voltage, Current, and Resistance. Without them you would not have a known value of resistance to use as a set point.

Resistance decade box is nothing but Resistance bank. It has various ranges of Resistance value ranging from few ohms to few mega ohms. In other terms, large value of resistance are fabricated in single box is said to be Resistance decade box.

we use it when we require a variable resistance in a circuit

It depends on what you want to accomplish. If you want to decrease the resistance in a circuit, you would place the box in parallel to some other resistor. If you want to increase the resistance in a circuit, you would place the box in series.