If I say, 'You are there', there is a sense in which you understand what I have just said. You, after all, know something that a monolingual Russian speaker knows. You know that I am speaking about some addressee, saying of him or her that he or she is located in some particular space, related in space and time to my egocentric location. This is all very well, but, at the same time, you seem to have no idea what that utterance means unless you grasp the very context in which it was uttered.

So do you understand it or do you not? I say you do not. You have no means for verifying its truth, beyond knowing the platitudinous:

'You are there' is true in context C iff the addressee in C is in the location denoted by the speaker of the utterance in C.

Does this suffice for understanding the indexical statement? Of course not. You can be said to understand it only if you know that:

The addressee of the utterance in C is that.

Since you do not know who that is, you do not know what I am saying.