Tack welds are welds that are used to connect two pieces of metal together before laying down the final weld or finishing weld. A finishing weld is the weld that covers the entire seam. Tack welds make the welding of large and complex projects like a car frame much easier because once you tack weld, the welder no longer has to hold the two pieces of metal together with their hands or magnets. A tack weld is ideal to hold the material together temporarily. Depending on what you are trying to connect and how you are connecting it tack welds are done in different ways. When you are taking two pieces of sheet metal and tacking them together you would want to have your welder perpendicular to the sheets and make a small circular motion. You should only do this for a few seconds and your tack welds should look something like the image below.
When you are joining a piece of metal perpendicular to the other piece you would weld similar to the first but this time you just go back and forth in a U shape against the perpendicular angle.
Tack welds are very important to almost any project and they are not very hard to learn so when you are learning how to weld make sure you have tack welds as a tool in your belt.