Pull Up Resistor

A Pull-up resistor is used to keep an input pin to an integrated circuit in a known state. Normally, if a "high-impedance" (input pin) to a device is not connected to anything, slight changes in the environment can cause the pin to read high or low with no real certainty. To absolutely make sure a pin is in a high state when nothing is connected to it, a "pull-up" resistor is used. The pull-up resistor acts exactly how it's name implies: it pulls the voltage on the input pin up to the logic level voltage (typically 5V or 3.3V). The most common use for these are on I2C lines, or with buttons / switches.

Example Circuit Button / Switch Input
The typical value for a pull-up resistor when used with a switch is 10KΩ or 20KΩ.

I2C Lines
The typical value for a pull-up resistor on an I2C line is approximately 1.8KΩ