what does High-Noise-Immunity mean?
It's right in front of you:

What they mean by &quot;high noise immunity&quot; is that it has a high (relative to TTL logic levels) threshold for switching between a low input level and a high input level. So small noise variations when the input voltage is low won't cause spurious switching.
In general, the bullet points on the front page of the datasheet are marketing and not technical. Sometimes they are useful for determining the intended application of a device, but you should generally ignore them and read the actual specification tables if you want to know how the device performs.