Grab a t-shirt, turn it inside out, and look at the seams. They will most likely be cocooned or wrapped in thread. That is what a serger does. Here are a couple of articles with descriptions, pictures, and the different options/stitches some sergers can do.
https://www.thesprucecrafts.com/unde...achine-2978199
https://www.mygoldenthimble.com/what...-need-to-know/
I'm like you, I rarely sew garments. I have a serger but I don't use it very often. Maybe once every 4-5 years.