High/Low Input Voltage Atmel At90USB128 Microcontroller
The key on the graph is the y axis label "threshold"

The table is telling you if you live within those limits, then the device will absolutely always work no matter the temp or phase of moon.

The graph is showing you the threshold where it typically flips, which varies by temp and device, but it'll probably be around there.

Imagine feeding in 0 volts and slowly ramping up.  Per the table with a 5 V supply you are being promised that anything less than 1.0 volts will be interpreted as a zero.  Per the graph if you continue increasing voltage above the "no mans land" of 1 volt, somewhere around 1.3 to 1.5 volts (depending on temp) the input will switch from 0 to 1.

Lets say you're designing a binary 1 or 0 input.  You need to guarantee that 0 voltage never rises above 1 volt if you want the device to think its a 0.

Lets say you're designing a capacitive RC time constant switch debouncer input, you might glancing at the table think the input will toggle 1/0 at 1.0 volts, but it actually does so at 1.5 volts.