It's hard to say without some context. Itineris means "of [a or the] journey", de natura is "about, or from, nature". The phrase De natura is common in Latin titles, and
… means "On the Nature [of] . . .". Itineris de natura could mean "On the Nature of [a/the] Journey", but the word order is unusual. Usually the genitive (the "of" word) either immediately precedes natura (Lucretius' De rerum natura , "On the Nature of Things") or immediately follows it (Cicero's De natura deorum , "On the Nature of the Gods"). Taking things the other way around, itineris de natura could mean ". . . of [a/the] journey away from Nature." (MORE)