Translation:I did not understand the text he sent me.
For those wondering about "that" vs. "which", here is a simplified explanation. When a restrictive clause follows, use "that". What is a restrictive clause? It is a clause that defines (and therefore restricts) the meaning. I.e., what text? The one that he sent me. When followed by a non-restrictive clause, use "which".
All right, thanks.
The DL translation only has "the text" which is a bit ambiguous.