Meld lines are formed when two separate flow fronts meets. These are preferred over weld lines as they will normally result in improved cosmetics and structure. In a meld line, the plastic comes together at an angle greater than 135 degrees. As compared to a weld line, the flow fronts come together at less than 135 degrees. If the cavity has unbalanced flow paths unnecessary meld lines can occur.
The figures below shows a part with a hole/window in it. The middle figure shows a gate location which results in a weld line. The figure on the right shows the formation of a meld line.