These are two logic levels foud in ICs. TTL and CMOS, both having different voltage ranges for representing logic 0 and logic 1.
TTL:
input
logic 0—-> 0 to 0.8V, logic 1
> 2 to 5V
output
logic 0—-> 0 to 0.5V, logic 1
> 2.7 to 5V
CMOS:
input
logic 0—-> 0 to 1.5V, logic 1
> 3.5 to 5V
output
logic 0—-> 0 to 0.05V, logic 1
> 4.95 to 5V