A serger finishes the edges of materials that like to fray or even unravel while you are making the garment OR after you start wearing it. A long time ago I used to zigzag the edges of my pattern pieces for this reason. The serger uses 4 separate threads and locks the edge for you, while trimming beyond the serged edge. It's very heavy duty but a real BXXTCH to rethread. DD used to sell them. The best thing is to knot the new threads on and pull those through the needle, giving it about 5 inches.
Sewing is a great skill, no matter what machine you have. =D