As you're welding, you're likely to encounter slag inclusions (foreign materials trapped in the weld metal at continuous or randomly spaced intervals). The most common is slag, a non-metallic solid material trapped in the weld or between the weld and the base metal.
One of the most common causes of slag inclusions is the presence of coatings on certain metals. Aluminum, for example, is often coated in aluminum oxide, which forms rapidly when aluminum is exposed to air. Those oxides can be trapped in your weld when you're working with aluminum, and the only solution is to clean the aluminum thoroughly to remove the oxides before you start to weld. (The same goes for other coatings on other metals.)
Copper backing bars are another common source of slag inclusions. The copper can melt away and become trapped in the finished weld. If you're using a copper backing bar and you want to avoid slag inclusions, keep a close eye on your penetration (weld depth) — don't make the weld too deep.