My words echo
Thus, in your mind.
But to what purpose
Disturbing the dust on a bowl of rose-leaves
I do not know.
As an illustration of the nothingness of the past and future, he uses himself writing Burnt Norton as an example. To Eliot, writing, your mind is no more than a possibility, about which he knows nothing. To you, reading, Eliot is someone long gone from the world, and all that remains are his words left behind like echoes. To me - writing this - both your moment as reader and Eliot's moment as writer literally do not exist.
The effect of his words on your mind is compared to 'disturbing the dust on a bowl of rose-leaves.' The rose and the leaves are important images in the rest of the poem. I take their meaning from how they are used later (I don't think you can get all this from that line on its own):
Rose represents ecstasy and innocence, and the perfection of this moment in time. The rose is a symbol in Neo-Platonism and Hermeticism. Yeats uses it a lot. It is used by Eliot to represent the act of rising up, that which rose.
Leaves represent the legacy of the past, or connectivity with the past. Although autumn leaves are a typical symbol of regret and nostalgia, that's incidental. The use of 'leaves' to mean what is left from the past is more to do with the sound of the word, like 'rose', a verb as well as a noun. What is left, and that which leaves.
It is funny that the symbol of the present is a verb in the past tense ('rose') and the symbol of the past is a verb in the present tense ('leaves'). But this is no more than the way language falls out, when you use a noun as a symbol of a verb.
The other word - dust - you might imagine such a word would come up a lot in Four Quartets, but without checking I don't think it does so much. But we understand it to mean neglected leftovers without vitality.
So, the poem disturbs the leavings of old ecstasies, or it gives false life to the left-over dust of his dead innocence. In any case, he is typically representing the melting away of time in the most gloomy and grumpy way.