The fluid parcel model states that a gas can be modelled as the sum of infinitesimal small parcels. According to this model, the friction of a gas with a solid surface (e.g. the wall of a pipe) results when the boundaries of a parcel "slide" against the solid surface which creates heat and decreases the parcel's velocity.
But a gas only consists of atoms or molecules. Microscopically, there are no boundaries onto which forces can act. Furthermore, all collisions between molecules and the wall are elastic with no kinetic energy lost.
So, how does the phenomenon emerge what we see as "friction" of a gas? How does the transition occur from no friction (microscopic) to friction (macroscopic)?