Digital I/O threshold voltages for ATmega32u4
LVTTL is Low Voltage Transistor-Transistor Logic. In general, the
term "TTL" is used for a digital signal. 
The "Low Voltage" part is special,
the ATmega32U4 switches from low to high (and vice versa) 
at low voltages.  
Just as it is written. For example, when the ATmega32U4 runs at 5V, 
then:
VILmax = 0.2 * 5 - 0.1 = 0.9 V.
VIHmin = 0.2 * 5 + 0.9 = 1.9 V. 
Between those values the input signal is seen as low or high,
but the exact voltage when it switches from low to high may
vary.
When the microcontroller is running at 5.0 V and is used 
within the specifications of the datasheet, then a voltage 
below 0.9 V is always seen as low and
a voltage above 1.9 V is always seen as high (guaranteed).
Those voltages is what a electronics designer must consider.


In the datasheet of the ATmega32U4 at page 403 is paragraph 30.7 "Pin Threshold and Hysteresis" with figure 30-22 and 30-23.
The figures show that at 25°C and 5V, the input switches from low to high typical at 1.55 V and it switched from high to low at maybe a slightly lower voltage.
The hysteresis is almost none and the voltage depends on VCC and the temperature.



(Thanks to Edgar Bonet for the correction/addition)