TTL Logic Levels
A majority of systems we use rely on either 3.3V or 5 V TTL Levels. TTL is an acronym for Transistor-Transistor Logic. It relies on circuits built from bipolar transistors to achieve switching and maintain logic states. Transistors are basically fancy-speak for electrically controlled switches. For any logic family, there are a number of threshold voltage levels to know. Below is an example for standard 5V TTL levels:

VOH — Minimum OUTPUT Voltage level a TTL device will provide for a HIGH signal.

VIH — Minimum INPUT Voltage level to be considered a HIGH.

VOL — Maximum OUTPUT Voltage level a device will provide for a LOW signal.

VIL — Maximum INPUT Voltage level to still be considered a LOW.
— Read on learn.sparkfun.com/tutorials/logic-levels/ttl-logic-levels