What is the difference between primary sources and secondary sources?

Iam told that hundreds if not thousands of these (armenians) were sent at once to the front ranks at the dardanelles, where death in a very short space of time is almost a certainty. the older men were then deported into the interior, while the women and children, when not carried off in an opposite direction, were left to shift for themselves as best they could. the terrible feature of this deportation up to date is that it has been carried out on such a basis as to render it practically impossible in thousands of cases that these families can ever again be reunited. not only wives and husbands, brothers and sisters, but even mothers and their little children have been dispersed in such a manner as to preclude practically all hope that they will ever see each other again. according to this excerpt, what does the author consider to be the worst part of the deportation during the armenian genocide?