“But when we look carefully at this pure being which constitutes
the essence of this certainty, and which this certainty
pronounces to be its truth, we see that much more is involved.”

the moment you see the object without altering was the real experience of that moment called truth. The object is nothing but a being, the moment of experiencing the being without the mind is termed as essence, this essence is called certainty. 
In simple term,
This certainty arises at that moment called ‘the real truth of experiencing that object’.

“An actual sense-certainty is not merely this pure immediacy,
but an instance of it. Among the countless differences cropping
up here we find in every case that the crucial one is that, in
sense-certainty,”

The experience of the object without altering called Sense-certainty. And this sense-certainty experience is just one experience to the mind. Normally after seeing the new object, the mind will bring a lot of comparison from your memory to understand but before such activity is the crucial experience of Sense-Certainty.

“Pure being at once splits up into what we have
called the two ‘Thises’, one ‘This’ as ‘I’, and the other ‘This’
as an object”.

Once the object is seen, there are two different experiences: one we are seeing the object and another object seen by us. Hence two event i.e. Two ‘Thises’. One This is ‘I’ and another This is ‘Object” i.e. ‘I and Object’.

“When we reflect on this difference, we find that
neither one nor the other is only immediately present in sense ..
certainty, but each is at the same time mediated:”

The whole experience of understanding is not only bringing immediacy but both the ‘This” communicating to each other.

“I have this certainty
through something else, viz. the thing; and it, similarly,
is in sense-certainty through something else, viz. through the ‘I’.

The formulation of Sense-Certainty was through an invisible act of “I experience the Object”  and the “Object experiencing with the help of I”