This is a word meant for yogi. I will not fully explain what it refers to – only because a Guru should do that
Basically it means that I am. That refers to God
Yogananda referred it to hungso technique. A peculiar sound produced by the breath with breathing out and in.
They are very powerful techniques. Having said that, there is a little more to it
Recently someone asked me where or which upanishad talks about sohum.
Yoga Sikha upanishad does. Here is a link to it – the translation
However without a Guru the full understanding and experience cannot be found
The full experience of sohum is similar to tattva Masi of veda. Which means that are though
A full description is in one other shastra. It can be found at 7 FB. I don’t understand much of it so can only leave it at that. That’s why I said best to hear from a Guru