Friction affects movement. Internally, it is represented by a fixed-point value that multiplies movement, therefore a high friction value results in low actual friction. A friction value of 1.0 (65536) correspond to no friction at all, movement being unaffected. Lower values result in greater friction, with a value of 0 being infinite friction and preventing all movement.
There are two types of friction: air friction for airborne objects, and ground friction for land-bound movement. Ground friction corresponds to a value of 59392 by default (90.625% of movement speed is retained), but this can be changed by using sector type 79:Sector_LowFriction (63744, about 97.27% of speed is retained), the Sector_SetFriction special (using arbitrary percentage values), or by tying a flat texture to a set friction through TERRAIN definition.
Air friction also depends on circumstances: there is some friction while flying with the fly or noclip2 cheats or the Wings of Wrath, but a different amount (which can be null, depending on sv_aircontrol) for jumping or falling actors (including player pawns) and floating monsters. Player flight friction is 60160 (retaining about 92% of speed), while otherwise air friction is 65536 (100% of speed) if sv_aircontrol is 1/256th or less, and computed from the formula 1.0004 - (0.941 × sv_aircontrol) otherwise. For example, with the default aircontrol value of 0.00390625, air friction is 65321 (retaining about 99.67% of speed).