The injection molding process is fairly simple, plastic resin is heated to its melting point and forced through the machine and into your mold to produce your plastic parts. The leading edge of the molten material is often the coolest point and the closest to solidifying.
When the molten plastic meets an obstruction it must travel around and meet at the other side. If the plastic has cooled too much during the injection process it can lead to knit lines in plastic parts when they meet past an obstruction.
ABS is the most common resin to be prone to knit lines.
Knit lines are most common downstream of holes created in parts where the molten material meets. While knit lines can be present it does not always mean the structural integrity of the plastic part is compromised.
If you are concerned about potential knit lines, turn to the mold flow of your part and address any design issues that can be easily modified, second review similar materials that may be less prone to show knit lines.
With good part design and a well designed mold, knit lines can often be reduced if not removed completely.