The page you link to calls this a "devotional song", suggesting that it's religious and that the entity "touching" the narrator is God. This makes sense as an interpretation of the poem:
As I walk along my way
I receive your touch
Now and then
But I don’t know how and when.
The narrator feels close to God at specific moments, but expresses uncertainty about exactly what those moments are.
Is it in the scent of an unknown flower
Or in the joy
I feel in the song of a travelling singer?
Joyful sensual experiences allow the narrator to feel the touch of the divine.
Do I receive your touch all on a sudden
When there is great sorrow
And my world is shaken
All the traces of my way are effaced
All the bonds are broken
And death deals a deadly blow?
On the other hand, the narrator is also thinking about tragic experiences, and whether they too enable him to feel the touch of the divine. I think the most likely interpretation here is that the narrator is turning to God for refuge in these times of great sorrow and death. (The answer to the question "Do I ..." might be no, since the whole poem has a tone of uncertainty, but the phrase "all on a sudden" suggests that it's something the narrator really does feel. An alternative explanation might be that this is about the power of God, or how an omnipotent entity who created everything can do bad as well as good, but I don't think this fits with the theme.)
I don't know.
The poem ends on the same note of uncertainty as it began.
How does this interpretation fit in with Tagore's religious views more generally? Well, we're lucky in that Tagore's works include The Religion of Man, a book of philosophical discussion on the nature of humanity and the divine, which is available to read on the Internet Archive. Near the beginning, he says:
This thought of God has not grown in my mind through any process of philosophical reasoning. On the contrary, it has followed the current of my temperament from early days until it suddenly flashed into my consciousness with a direct vision. The experience which I have described in one of the chapters which follow convinced me that on the surface of our being we have the ever-changing phases of the individual self, but in the depth there dwells the Eternal Spirit of human unity beyond our direct knowledge. [...]
During the discussion of my own religious experience I have expressed my belief that the first stage of my realization was through my feeling of intimacy with Nature - not that nature which has its channel of information for our mind and physical relationship with our living body, but that which satisfies our personality with manifestations that make our life rich and stimulate our imagination in their harmony of forms, colours, sounds and movements. It is not that world which vanishes into abstract symbols behind its own testimony to Science, but that which lavishly displays its wealth of reality to our personal self having its own perpetual reaction upon our human nature.
Seeing a connection with Nature as a religious experience is certainly consistent with this poem's description of receiving the touch of God in the scent of a flower or the song of a singer. But, in light of these religious views as expressed by Tagore, perhaps we should interpret "God" in his way as a sort of ideal representation of humanity as a whole, "the divinity of Man the Eternal" as he writes in his book. Whether at moments of joy or moments of sorrow, the narrator is a part of humanity, and can be reminded of his own connection with the universal human spirit, which is Tagore's interpretation of God.