The press secretary explained the contents of the treaty to the press corps.

Literal

Press-officer [subject-が] treaty [possessive-の] contents [object-を] press-corps [to-に] explained

報道担当官 = press + in-charge + official. 報道陣 = press + formation — 陣 gives a collective sense. 条約 = treaty. The sentence is dense with formal kanji compounds typical of news reporting.