Unlike analogue circuits, elements in digital logic circuits are in states of either "high" or "low" (usually corresponding to "true" and "false", or "1" and "0"). In most cases, highs and lows are exhibited as voltage levels on the input and output pins of the integrated circuit. Depending on the technology, this voltage level will vary.
For Transistor-Transistor Logic (TTL) circuits, which is the most popular kind used in electronic projects, high is anything above 2V, and low is anything below 0.7V. In these circuits, it is very common to use +5V for high and ground for low. If a pin is not connected (neither connected to the high voltage nor grounded), then it is floating. TTL chips usually go high by themselves when floating, but don't count on it. Inputs should never be floating; always have them connected to something.
Active Low Input
An active low input means that it is "on" when in input is low, and "off" when the input is high. In schematic diagrams, it is often denoted by a "bubble" at the input pin.
High usually corresponds to the binary "1," the "on" state, and "true" or "1" in Boolean logic.
Low usually corresponds to the binary "0," the "off" sate, and "false" or "0" in Boolean logic.