It would be nonphysical, spiritual travel. If you'd like, you can take a look at Christopher Penczak's book, "Ascension Magick" where he discusses his personal understanding and gnosis involving the Merkaba (or as he calls it, the "Light Body"). It is heavily oriented towards UPG and New Age paradigms, as a fair warning.
Beyond that, all I personally know of it is that the word and concept originate in Jewish Mysticism.
The best I can do to explain the difference between Merkaba Travel and Astral travel, is that Merkaba is a complete soul movement technique.
When a person Astral Travels, only the largest portion of their soul leaves the body to travel and leaves behind a lesser portion as a tether, often appearing to look like a umbilical cord from your soul's form to your physical body. The general knowledge concerning that cord is that if it were to be severed, the possibility exists that you won't be able to return to your body. In other words, the body dies.
Merkaba Travel utilizes the complete system of spiritual energy that circulates your body. This can take many different forms, based on your beliefs and specific culture, but the general idea of it is that your entire spirit becomes a vehicle for ascension.
Because of this, there is no tether involved. Using the Merkaba is a direct act of ascension and leaving behind the physical wants and desires. This is why there is the understanding that using the Merkaba to it's fullest potential, will lead a soul to completely leave the physical plane and ascend, causing the body to die.
I am not the best authority on this though. It's best to cross-check this with references, but this is how I've come to understand the concept of Merkaba.