Currently I am working with
Pull up and pull down registers and trying to understand what does it mean?
But could not able to understand. I searched in Wikipedia but still confused.
This picture attempts to show what pullup and pulldown resistors do:
Start with the first diagram, and ask what the voltage is where I've drawn the dotted arrow. I've put "some component" to indicate something else is connected: usually this would be a switch - either a hardware switch or a transistor acting as a switch.
When the switch is closed, the resistance between the dotted arrow and the ground is zero (because a closed switch has zero resistance), so the voltage drop across the resistor is $V$ and the current flowing through the resistor is given by $I = V/R$. The voltage at the dotted arrow is zero.
When the switch is open no current flows. Now the voltage drop across the resistor is zero, because $V = IR$, so the voltage at the dotted arrow is +V. This is why it's called a pullup resistor, because when the switch is open it pulls up the voltage to be +V.
The second diagram shows a pulldown resistor. This is very similar but in this case when the switch is open the resistor pulls down the voltage at the dotted arrow to zero, which is why it's called a pulldown resistor.