Rashi: It is an expression of swallowing, like "Sha'afah Ru'ach" (Yirmeyah 2:24).

Radak: This is a metaphor, like "va'Eftach Es Pi va'Ya'achileni Es ha'Megilah" (Yechezkel 3:2). I opened my mouth and inhaled. Just like one who is hungry and desires food, when they bring it to him, amidst desire he opens his mouth and inhales before eating.

Malbim: Torah is compared to bread and to water. One cannot live without them; they are food of the body. So Mitzvos are food of the Nefesh. David says that for him, Mitzvos are like air. Bread and water, one can live without them for some days. One cannot live without air even for a moment! I constantly open my mouth and inhale this spiritual air. I.e. at every moment I do a Mitzvah; it is inhaling spiritual life.

Radak: This does not appear elsewhere in Tanach. It is like Ta'avti (I desired).