It means that sound insulation is used to "isolate" your hearing apparatus from ambient noise - usually the airplane noise is what is discussed here, of course.

Earplugs are sound isolation.

The competing technology is "noise cancellation" in which ambient noise is sampled and actively, electronically "whited out".

The noise isolation devices - notably Shure - were developed at first for musicians who needed a monitor source for what they and their fellows were playing while getting some protection against the loud volumes one is exposed to while standing in front of a huge array of speakers. For that reason, attention to the quality of sound was a factor from the get go.

Bose and others used the kind of sound cancellation that has been used in radio headsets in aviation where the primary consideration has been protection from noise and being able merely to hear voice communication from the ground via radio or from people on board via intercom. Sound reproduction to in the full ranges required to enjoy the nuance of music was not a consideration at first.

I first bought a set of Bose. They seemed to "deaden" much of what I like to listen to. I tried a set of Shures in the middle range (E3's I think) side by side during a long flight. I went home and sole the Bose that I had owned less than three months.

I now use a set of E5's - since replaced by the new series - that were at the top of their line when I got them. Heaven!

The sound isolation does not exclude all cabin/engine noise by themselves. But with music at even a low-moderate level, that external noise is gone for me.

I have never tried the Etymonics. I hear they are good, too.

Primary caveat is that the isolating system are typically "in ear". This means inserting the ear buds fairly deeply to get a good seal. Some find that uncomfortable. Some, like me, find that 15 - 20 hours of constant use is just fine.