Attaches the handlebars to the bike, or, more precisely, to the steerer tube. Stems are designed differently for different headset types (i.e. threaded or threadless). They are also sized to suit different diameter handlebars and in the case of threadless setups, different diameter steerer tubes.
Stems play a key role in setting your riding position. You can adjust the reach to the handlebars by changing the stem’s length. Separately, you can move the stem vertically to change your riding position. Stems for road bikes typically come in lengths ranging from 80 to 140mm. Mountain bikes have moved to significantly shorter stems than this. If you need a stem much shorter than 80mm on a road bike, that is a sign the bike doesn’t fit you. All else equal, a shorter stem will make for quicker handling. If you are buying a new bike and you know you need a different stem length, the bike store should be willing to trade your stem for something they have in stock.
Another characteristic of stems is their angle. Threadless stems commonly come in angles around -10 degrees, sometimes more. You can flip a threadless stem so that it increases the height of the bar. There are also angle-adjustable stems made, so the rider may change bar positions without removing the handlebars or changing the stem, although their length typically can’t be adjusted.
Trifox also has some stem accessories: Road Bicycle Handlebar Stem ABS100 is made of 6061 Aluminum Alloy, with 7° up rise, suit for 31.8 Handlebar and 28.6mm Fork, for Mountain Bike (MTB) / Road Bike.