What is a Logic Level?
Put simply, a logic level is a specific voltage or a state in which a signal can exist. We often refer to the two states in a digital circuit to be ON or OFF. Represented in binary, an ON translates to a binary 1, and an OFF translates to a binary 0. In Arduino, we call these signals HIGH or LOW, respectively. There are several different technologies that have evolved over the past 30 years in electronics to define the various voltage levels.
Logic 0 or Logic 1
Digital electronics rely on binary logic to store, process, and transmit data or information. Binary Logic refers to one of two states -- ON or OFF. This is commonly translated as a binary 1 or binary 0. A binary 1 is also referred to as a HIGH signal and a binary 0 is referred to as a LOW signal.
The strength of a signal is typically described by its voltage level. How is a logic 0 (LOW) or a logic 1 (HIGH) defined? Manufacturers of chips generally define these in their spec sheets. The most common standard is TTL or Transistor-Transistor Logic.