The Hebrew word aph (אַף) functions as a conjunctive adverb meaning also, moreover, yea, or even. It intensifies or adds to what has been said. It is extremely common in Hebrew poetry and discourse, often appearing at the beginning of clauses to emphasize an additional or surprising point.
The word aph often introduces the climax or intensification of an argument in Hebrew rhetoric. When the Psalms say 'Yea, though I walk through the valley...' the aph emphasizes the extreme case — even in death's shadow, God is present. This small word carries great theological weight: it asserts that God's care extends even to the worst circumstances, that His promises hold even in unlikely situations.